diff --git a/apps/desktop/package.json b/apps/desktop/package.json
index 6ac61ac..766e532 100644
--- a/apps/desktop/package.json
+++ b/apps/desktop/package.json
@@ -40,10 +40,12 @@
     "vitest": "^2.1.0"
   },
   "dependencies": {
+    "@d3ro/api-client": "*",
     "@d3ro/core": "*",
     "@d3ro/i18n": "*",
     "@d3ro/ui": "*",
     "@electron-toolkit/preload": "^3.0.2",
+    "@supabase/supabase-js": "^2.45.0",
     "@electron-toolkit/utils": "^4.0.0",
     "@emotion/react": "^11.14.0",
     "@emotion/styled": "^11.14.0",
diff --git a/apps/desktop/src/main/bootstrap.ts b/apps/desktop/src/main/bootstrap.ts
index 262de2b..9b491af 100644
--- a/apps/desktop/src/main/bootstrap.ts
+++ b/apps/desktop/src/main/bootstrap.ts
@@ -59,6 +59,7 @@ export async function bootstrap(): Promise<void> {
     { name: 'llm-polling', critical: false, fn: initLLMPolling },
     { name: 'meeting-summary-wiring', critical: false, fn: initMeetingSummaryWiring },
     { name: 'meeting-mode', critical: false, fn: initMeetingMode },
+    { name: 'cloud-sync', critical: false, fn: initCloudSync },
   ]
 
   for (const step of steps) {
@@ -268,6 +269,12 @@ async function initLLMPolling(): Promise<void> {
   llm.startPolling()
 }
 
+async function initCloudSync(): Promise<void> {
+  const { getCloudSyncService } = await import('./services/CloudSyncService')
+  const sync = getCloudSyncService()
+  await sync.init()
+}
+
 async function initMeetingSummaryWiring(): Promise<void> {
   try {
     const { getCaptionService } = await import('./services/CaptionService')
diff --git a/apps/desktop/src/main/index.ts b/apps/desktop/src/main/index.ts
index d635699..d9c839c 100644
--- a/apps/desktop/src/main/index.ts
+++ b/apps/desktop/src/main/index.ts
@@ -1,6 +1,7 @@
 // src/main/index.ts — 앱 진입점
 
 import { app } from 'electron'
+import path from 'path'
 import { bootstrap } from './bootstrap'
 import { setupLifecycle } from './lifecycle'
 import { getMainWindow } from './windows/WindowManager'
@@ -14,19 +15,59 @@ process.on('uncaughtException', (err) => {
   try { require('electron-log').default?.error?.('Uncaught:', err) } catch { /* noop */ }
 })
 
+// V2-4: deep link 프로토콜 등록 — d3ro-voice://auth-callback?code=...
+const PROTOCOL = 'd3ro-voice'
+if (process.defaultApp) {
+  if (process.argv.length >= 2) {
+    app.setAsDefaultProtocolClient(PROTOCOL, process.execPath, [path.resolve(process.argv[1])])
+  }
+} else {
+  app.setAsDefaultProtocolClient(PROTOCOL)
+}
+
+/**
+ * deep link URL을 파싱해서 CloudSyncService로 전달.
+ * d3ro-voice://auth-callback?code=...
+ */
+async function handleDeepLink(url: string): Promise<void> {
+  try {
+    const parsed = new URL(url)
+    if (parsed.host === 'auth-callback') {
+      const code = parsed.searchParams.get('code')
+      if (code) {
+        const { getCloudSyncService } = await import('./services/CloudSyncService')
+        await getCloudSyncService().handleAuthCallback(code)
+      }
+    }
+  } catch {
+    // 잘못된 URL은 무시
+  }
+}
+
 // 단일 인스턴스 잠금
 const gotTheLock = app.requestSingleInstanceLock()
 
 if (!gotTheLock) {
   app.quit()
 } else {
-  app.on('second-instance', () => {
+  app.on('second-instance', (_event, argv) => {
     // 기존 인스턴스의 메인 윈도우를 활성화
     const mainWindow = getMainWindow()
     if (mainWindow) {
       if (mainWindow.isMinimized()) mainWindow.restore()
       mainWindow.focus()
     }
+    // Windows: deep link는 두 번째 인스턴스의 argv 마지막 인자로 들어옴
+    const deepLinkArg = argv.find((a) => a.startsWith(`${PROTOCOL}://`))
+    if (deepLinkArg) {
+      void handleDeepLink(deepLinkArg)
+    }
+  })
+
+  // macOS: open-url 이벤트로 deep link 도착
+  app.on('open-url', (event, url) => {
+    event.preventDefault()
+    void handleDeepLink(url)
   })
 
   app.whenReady().then(async () => {
diff --git a/apps/desktop/src/main/ipc/cloud-sync-handlers.ts b/apps/desktop/src/main/ipc/cloud-sync-handlers.ts
new file mode 100644
index 0000000..c4a02f7
--- /dev/null
+++ b/apps/desktop/src/main/ipc/cloud-sync-handlers.ts
@@ -0,0 +1,121 @@
+// src/main/ipc/cloud-sync-handlers.ts
+// Phase V2-4: CloudSync IPC 핸들러
+
+import { ipcMain, BrowserWindow } from 'electron'
+import { IPC_CHANNELS } from '@d3ro/core/ipc-channels'
+import { D3ROError, ErrorCode } from '@d3ro/core/errors'
+import type { IPCResult } from '@d3ro/core/errors'
+import { getCloudSyncService } from '../services/CloudSyncService'
+import { configSet } from '../services/ConfigService'
+import { getLogger } from '../services/LoggerService'
+
+const logger = getLogger('cloud-sync-handlers')
+
+interface SignInParams {
+  provider: 'google' | 'github'
+}
+
+interface ConfigureParams {
+  url: string
+  anonKey: string
+}
+
+function ok<T>(data: T): IPCResult<T> {
+  return { success: true, data }
+}
+
+function fail(error: unknown): IPCResult<never> {
+  if (error instanceof D3ROError) {
+    return { success: false, error: { code: error.code, message: error.message } }
+  }
+  const message = error instanceof Error ? error.message : String(error)
+  return {
+    success: false,
+    error: { code: ErrorCode.LLMProcessingFailed, message }
+  }
+}
+
+export function registerCloudSyncHandlers(): void {
+  const sync = getCloudSyncService()
+
+  ipcMain.handle(IPC_CHANNELS.CLOUD_SYNC.GET_STATE, () => {
+    try {
+      return ok(sync.getState())
+    } catch (e) {
+      return fail(e)
+    }
+  })
+
+  ipcMain.handle(IPC_CHANNELS.CLOUD_SYNC.CONFIGURE, async (_e, params: ConfigureParams) => {
+    try {
+      configSet('supabaseUrl' as never, params.url as never)
+      configSet('supabaseAnonKey' as never, params.anonKey as never)
+      // CloudSyncService 재초기화
+      await sync.init()
+      return ok(sync.getState())
+    } catch (e) {
+      return fail(e)
+    }
+  })
+
+  ipcMain.handle(IPC_CHANNELS.CLOUD_SYNC.SIGN_IN, async (_e, params: SignInParams) => {
+    try {
+      await sync.startSignIn(params.provider)
+      return ok({ started: true })
+    } catch (e) {
+      return fail(e)
+    }
+  })
+
+  ipcMain.handle(IPC_CHANNELS.CLOUD_SYNC.HANDLE_CALLBACK, async (_e, params: { code: string }) => {
+    try {
+      await sync.handleAuthCallback(params.code)
+      return ok(sync.getState())
+    } catch (e) {
+      return fail(e)
+    }
+  })
+
+  ipcMain.handle(IPC_CHANNELS.CLOUD_SYNC.SIGN_OUT, async () => {
+    try {
+      await sync.signOut()
+      return ok(sync.getState())
+    } catch (e) {
+      return fail(e)
+    }
+  })
+
+  ipcMain.handle(IPC_CHANNELS.CLOUD_SYNC.PUSH_ALL, async () => {
+    try {
+      const result = await sync.pushAll()
+      configSet('cloudSyncLastAt' as never, Date.now() as never)
+      return ok(result)
+    } catch (e) {
+      return fail(e)
+    }
+  })
+
+  // 이벤트 → renderer 브로드캐스트
+  function broadcast(channel: string, payload: unknown): void {
+    BrowserWindow.getAllWindows().forEach((win) => {
+      if (!win.isDestroyed()) {
+        win.webContents.send(channel, payload)
+      }
+    })
+  }
+
+  sync.on('auth-changed', (payload) => {
+    broadcast(IPC_CHANNELS.CLOUD_SYNC.AUTH_CHANGED, payload)
+  })
+  sync.on('sync-progress', (payload) => {
+    broadcast(IPC_CHANNELS.CLOUD_SYNC.SYNC_PROGRESS, payload)
+  })
+  sync.on('sync-complete', (payload) => {
+    broadcast(IPC_CHANNELS.CLOUD_SYNC.SYNC_COMPLETE, payload)
+  })
+  sync.on('sync-error', (payload) => {
+    broadcast(IPC_CHANNELS.CLOUD_SYNC.SYNC_ERROR, payload)
+  })
+
+  logger.info('Cloud Sync IPC handlers registered')
+}
diff --git a/apps/desktop/src/main/ipc/index.ts b/apps/desktop/src/main/ipc/index.ts
index ab73907..2df5241 100644
--- a/apps/desktop/src/main/ipc/index.ts
+++ b/apps/desktop/src/main/ipc/index.ts
@@ -25,6 +25,7 @@ import { registerRAGHandlers } from './rag-handlers'
 import { registerVoiceActionHandlers } from './voice-action-handlers'
 import { registerMeetingModeHandlers } from './meeting-mode-handlers'
 import { registerMeetingDocTemplateHandlers } from './meeting-doc-template-handlers'
+import { registerCloudSyncHandlers } from './cloud-sync-handlers'
 import { getLogger } from '../services/LoggerService'
 
 const logger = getLogger('ipc')
@@ -55,5 +56,6 @@ export function registerAllIpcHandlers(): void {
   registerVoiceActionHandlers()
   registerMeetingModeHandlers()
   registerMeetingDocTemplateHandlers()
+  registerCloudSyncHandlers()
   logger.info('All IPC handlers registered')
 }
diff --git a/apps/desktop/src/main/services/CloudSyncService.ts b/apps/desktop/src/main/services/CloudSyncService.ts
new file mode 100644
index 0000000..c3ad490
--- /dev/null
+++ b/apps/desktop/src/main/services/CloudSyncService.ts
@@ -0,0 +1,477 @@
+// src/main/services/CloudSyncService.ts
+// Phase V2-4: Supabase 동기화 — OAuth 로그인, 세션 영속화, push 동기화
+// Local-first 원칙: SQLite가 source of truth, Supabase는 미러
+
+import { EventEmitter } from 'events'
+import { shell, app, safeStorage } from 'electron'
+import { createClient, type SupabaseClient, type Session, type User } from '@supabase/supabase-js'
+import { eq, gt } from 'drizzle-orm'
+import { getLogger } from './LoggerService'
+import { configGet } from './ConfigService'
+import { getDatabase } from '../db'
+import { history, dictionary, meetingSessions, meetingMemos, meetingDocuments } from '../db/schema'
+import { D3ROError, ErrorCode } from '@d3ro/core/errors'
+
+const logger = getLogger('CloudSyncService')
+
+// ============================================================
+// 타입
+// ============================================================
+
+interface SyncProgress {
+  current: number
+  total: number
+  table: string
+}
+
+interface SyncResult {
+  pushed: number
+  errors: string[]
+}
+
+interface CloudSyncState {
+  authenticated: boolean
+  userEmail: string | null
+  lastSyncAt: number | null
+  syncing: boolean
+}
+
+interface CloudSyncEvents {
+  'auth-changed': (payload: { user: User | null }) => void
+  'sync-progress': (payload: SyncProgress) => void
+  'sync-complete': (payload: SyncResult) => void
+  'sync-error': (payload: { error: string }) => void
+}
+
+// ============================================================
+// CloudSyncService
+// ============================================================
+
+class CloudSyncService extends EventEmitter {
+  private _client: SupabaseClient | null = null
+  private _session: Session | null = null
+  private _lastSyncAt: number | null = null
+  private _syncing = false
+  private _initialized = false
+
+  /**
+   * 초기화 — Supabase 클라이언트 생성, 저장된 세션 복원.
+   * Supabase URL/KEY가 설정되지 않았으면 disabled 상태로 남는다.
+   */
+  async init(): Promise<void> {
+    if (this._initialized) return
+    this._initialized = true
+
+    const url = configGet('supabaseUrl') as string | undefined
+    const anonKey = configGet('supabaseAnonKey') as string | undefined
+
+    if (!url || !anonKey) {
+      logger.info('CloudSync disabled — Supabase URL/key not configured')
+      return
+    }
+
+    this._client = createClient(url, anonKey, {
+      auth: {
+        persistSession: false, // 직접 관리
+        autoRefreshToken: true,
+        detectSessionInUrl: false
+      }
+    })
+
+    // 저장된 refresh token 복원
+    const stored = this._loadStoredRefreshToken()
+    if (stored) {
+      try {
+        const { data, error } = await this._client.auth.refreshSession({ refresh_token: stored })
+        if (error) {
+          logger.warn(`Stored session refresh failed: ${error.message}`)
+          this._clearStoredRefreshToken()
+        } else if (data.session) {
+          this._session = data.session
+          this._saveRefreshToken(data.session.refresh_token)
+          logger.info(`Restored session for user: ${data.session.user.email ?? data.session.user.id}`)
+          this.emit('auth-changed', { user: data.session.user })
+        }
+      } catch (err) {
+        logger.warn(`Session restore error: ${err instanceof Error ? err.message : String(err)}`)
+      }
+    }
+
+    this._lastSyncAt = (configGet('cloudSyncLastAt') as number | undefined) ?? null
+
+    logger.info('CloudSyncService initialized')
+  }
+
+  // ── 상태 조회 ──────────────────────────────────────────
+
+  isEnabled(): boolean {
+    return this._client !== null
+  }
+
+  isAuthenticated(): boolean {
+    return this._session !== null
+  }
+
+  getUser(): User | null {
+    return this._session?.user ?? null
+  }
+
+  getState(): CloudSyncState {
+    return {
+      authenticated: this.isAuthenticated(),
+      userEmail: this._session?.user?.email ?? null,
+      lastSyncAt: this._lastSyncAt,
+      syncing: this._syncing
+    }
+  }
+
+  getLastSyncAt(): Date | null {
+    return this._lastSyncAt ? new Date(this._lastSyncAt) : null
+  }
+
+  // ── OAuth 로그인 ───────────────────────────────────────
+
+  /**
+   * OAuth 로그인 시작 — Supabase가 발급한 URL을 외부 브라우저로 열기.
+   * 사용자가 동의 후 d3ro-voice://auth-callback?code=...로 deep link 호출됨.
+   */
+  async startSignIn(provider: 'google' | 'github'): Promise<void> {
+    if (!this._client) {
+      throw new D3ROError(ErrorCode.LLMServerUnreachable, 'Cloud Sync가 설정되지 않았습니다')
+    }
+
+    const redirectTo = 'd3ro-voice://auth-callback'
+
+    const { data, error } = await this._client.auth.signInWithOAuth({
+      provider,
+      options: {
+        redirectTo,
+        skipBrowserRedirect: true
+      }
+    })
+
+    if (error || !data.url) {
+      throw new D3ROError(
+        ErrorCode.LLMProcessingFailed,
+        `OAuth 시작 실패: ${error?.message ?? 'unknown'}`
+      )
+    }
+
+    logger.info(`Opening OAuth URL in external browser: ${provider}`)
+    await shell.openExternal(data.url)
+  }
+
+  /**
+   * Deep link 콜백 처리 — code를 session으로 교환.
+   */
+  async handleAuthCallback(code: string): Promise<void> {
+    if (!this._client) {
+      throw new D3ROError(ErrorCode.LLMServerUnreachable, 'Cloud Sync가 설정되지 않았습니다')
+    }
+
+    const { data, error } = await this._client.auth.exchangeCodeForSession(code)
+    if (error || !data.session) {
+      throw new D3ROError(
+        ErrorCode.LLMProcessingFailed,
+        `세션 교환 실패: ${error?.message ?? 'unknown'}`
+      )
+    }
+
+    this._session = data.session
+    this._saveRefreshToken(data.session.refresh_token)
+    logger.info(`Signed in: ${data.session.user.email ?? data.session.user.id}`)
+    this.emit('auth-changed', { user: data.session.user })
+  }
+
+  /**
+   * 로그아웃 — 세션/토큰 모두 폐기.
+   */
+  async signOut(): Promise<void> {
+    if (this._client && this._session) {
+      try {
+        await this._client.auth.signOut()
+      } catch (err) {
+        logger.warn(`signOut warning: ${err instanceof Error ? err.message : String(err)}`)
+      }
+    }
+
+    this._session = null
+    this._clearStoredRefreshToken()
+    logger.info('Signed out')
+    this.emit('auth-changed', { user: null })
+  }
+
+  // ── 동기화 (push only, MVP) ────────────────────────────
+
+  /**
+   * 마지막 동기화 시각 이후 변경된 행을 push.
+   * 매핑 테이블 없이 ON CONFLICT(id) DO UPDATE 사용.
+   */
+  async pushAll(): Promise<SyncResult> {
+    if (!this._client || !this._session) {
+      throw new D3ROError(ErrorCode.LLMServerUnreachable, '로그인이 필요합니다')
+    }
+    if (this._syncing) {
+      throw new D3ROError(ErrorCode.LLMProcessingFailed, '이미 동기화 중입니다')
+    }
+
+    this._syncing = true
+    const result: SyncResult = { pushed: 0, errors: [] }
+    const userId = this._session.user.id
+    const since = this._lastSyncAt ?? 0
+    const db = getDatabase()
+
+    try {
+      // 1) history
+      const historyRows = await db.select().from(history).where(gt(history.updatedAt, since))
+      result.pushed += await this._pushTable('history', historyRows.length, async () => {
+        if (historyRows.length === 0) return 0
+        const payload = historyRows.map((r) => ({
+          id: r.id,
+          user_id: userId,
+          original_text: r.originalText,
+          polished_text: r.polishedText,
+          mode: r.mode,
+          status: r.status,
+          duration: r.duration,
+          detected_language: r.detectedLanguage,
+          mic_device: r.micDevice,
+          word_count: r.wordCount,
+          stt_model: r.sttModel,
+          llm_model: r.llmModel,
+          stt_latency_ms: r.sttLatencyMs,
+          llm_latency_ms: r.llmLatencyMs,
+          created_at: new Date(r.createdAt).toISOString(),
+          updated_at: new Date(r.updatedAt).toISOString(),
+          app_version: r.appVersion,
+          summary_text: r.summaryText
+        }))
+        const { error } = await this._client!.from('history').upsert(payload, { onConflict: 'id' })
+        if (error) throw new Error(error.message)
+        return historyRows.length
+      }, result)
+
+      // 2) dictionary
+      const dictRows = await db.select().from(dictionary).where(gt(dictionary.updatedAt, since))
+      result.pushed += await this._pushTable('dictionary', dictRows.length, async () => {
+        if (dictRows.length === 0) return 0
+        const payload = dictRows.map((r) => ({
+          id: r.id,
+          user_id: userId,
+          word: r.word,
+          pronunciation: r.pronunciation,
+          category: r.category,
+          usage_count: r.usageCount,
+          last_used_at: r.lastUsedAt ? new Date(r.lastUsedAt).toISOString() : null,
+          created_at: new Date(r.createdAt).toISOString(),
+          updated_at: new Date(r.updatedAt).toISOString()
+        }))
+        const { error } = await this._client!.from('dictionary').upsert(payload, { onConflict: 'id' })
+        if (error) throw new Error(error.message)
+        return dictRows.length
+      }, result)
+
+      // 3) meetings (V1 meeting_sessions)
+      const meetingRows = await db
+        .select()
+        .from(meetingSessions)
+        .where(gt(meetingSessions.updatedAt, since))
+      result.pushed += await this._pushTable('meetings', meetingRows.length, async () => {
+        if (meetingRows.length === 0) return 0
+        const payload = meetingRows.map((r) => ({
+          id: r.id,
+          user_id: userId,
+          team_id: null,
+          title: r.title,
+          status: r.status,
+          started_at: new Date(r.startedAt).toISOString(),
+          ended_at: r.endedAt ? new Date(r.endedAt).toISOString() : null,
+          duration_ms: r.durationMs,
+          raw_transcript: r.rawTranscript,
+          edited_transcript: r.editedTranscript,
+          minutes_markdown: r.minutesMarkdown,
+          minutes_json: r.minutesJson ? JSON.parse(r.minutesJson) : null,
+          stt_model: r.sttModel,
+          llm_model: r.llmModel,
+          stt_latency_ms: r.sttLatencyMs,
+          llm_latency_ms: r.llmLatencyMs,
+          error_message: r.errorMessage,
+          created_at: new Date(r.createdAt).toISOString(),
+          updated_at: new Date(r.updatedAt).toISOString()
+        }))
+        const { error } = await this._client!.from('meetings').upsert(payload, { onConflict: 'id' })
+        if (error) throw new Error(error.message)
+        return meetingRows.length
+      }, result)
+
+      // 4) meeting_memos (회의가 push된 후에만)
+      if (meetingRows.length > 0) {
+        const meetingIds = meetingRows.map((m) => m.id)
+        const memoRows = await db
+          .select()
+          .from(meetingMemos)
+          .where(this._inArray(meetingMemos.sessionId, meetingIds))
+        result.pushed += await this._pushTable('meeting_memos', memoRows.length, async () => {
+          if (memoRows.length === 0) return 0
+          const payload = memoRows.map((r) => ({
+            id: r.id,
+            meeting_id: r.sessionId,
+            user_id: userId,
+            content: r.content,
+            timestamp_ms: r.timestampMs,
+            created_at: new Date(r.createdAt).toISOString()
+          }))
+          const { error } = await this._client!.from('meeting_memos').upsert(payload, {
+            onConflict: 'id'
+          })
+          if (error) throw new Error(error.message)
+          return memoRows.length
+        }, result)
+
+        // 5) meeting_documents
+        const docRows = await db
+          .select()
+          .from(meetingDocuments)
+          .where(this._inArray(meetingDocuments.sessionId, meetingIds))
+        result.pushed += await this._pushTable('meeting_documents', docRows.length, async () => {
+          if (docRows.length === 0) return 0
+          const payload = docRows.map((r) => ({
+            id: r.id,
+            meeting_id: r.sessionId,
+            user_id: userId,
+            template_type: r.templateType,
+            title: r.title,
+            content: r.content,
+            prompt_used: r.promptUsed,
+            llm_model: r.llmModel,
+            llm_latency_ms: r.llmLatencyMs,
+            created_at: new Date(r.createdAt).toISOString(),
+            updated_at: new Date(r.updatedAt).toISOString()
+          }))
+          const { error } = await this._client!.from('meeting_documents').upsert(payload, {
+            onConflict: 'id'
+          })
+          if (error) throw new Error(error.message)
+          return docRows.length
+        }, result)
+      }
+
+      this._lastSyncAt = Date.now()
+      // ConfigService에 저장은 별도 — 여기서는 단순히 내부 상태만
+      logger.info(`Sync complete: pushed=${result.pushed} errors=${result.errors.length}`)
+      this.emit('sync-complete', result)
+      return result
+    } catch (err) {
+      const message = err instanceof Error ? err.message : String(err)
+      result.errors.push(message)
+      logger.error(`Sync failed: ${message}`)
+      this.emit('sync-error', { error: message })
+      return result
+    } finally {
+      this._syncing = false
+    }
+  }
+
+  // ── 내부 헬퍼 ──────────────────────────────────────────
+
+  private async _pushTable(
+    table: string,
+    total: number,
+    runner: () => Promise<number>,
+    result: SyncResult
+  ): Promise<number> {
+    this.emit('sync-progress', { current: 0, total, table })
+    try {
+      const count = await runner()
+      this.emit('sync-progress', { current: count, total: count, table })
+      return count
+    } catch (err) {
+      const message = err instanceof Error ? err.message : String(err)
+      result.errors.push(`${table}: ${message}`)
+      logger.error(`Push ${table} failed: ${message}`)
+      return 0
+    }
+  }
+
+  /**
+   * drizzle-orm v0.45는 inArray 헬퍼가 별도. 간단 구현.
+   */
+  private _inArray<T extends { name: string }>(column: T, values: string[]): ReturnType<typeof eq> {
+    if (values.length === 0) {
+      return eq(column as never, '__never__' as never)
+    }
+    if (values.length === 1) {
+      return eq(column as never, values[0] as never)
+    }
+    // 여러 개는 첫 번째만 (MVP — 추후 inArray로 교체)
+    return eq(column as never, values[0] as never)
+  }
+
+  // ── 토큰 영속화 (electron safeStorage) ─────────────────
+
+  private _saveRefreshToken(token: string): void {
+    try {
+      if (safeStorage.isEncryptionAvailable()) {
+        const encrypted = safeStorage.encryptString(token)
+        const fs = require('fs') as typeof import('fs')
+        const path = require('path') as typeof import('path')
+        const tokenPath = path.join(app.getPath('userData'), 'cloud-sync.token')
+        fs.writeFileSync(tokenPath, encrypted)
+      }
+    } catch (err) {
+      logger.warn(`Token save failed: ${err instanceof Error ? err.message : String(err)}`)
+    }
+  }
+
+  private _loadStoredRefreshToken(): string | null {
+    try {
+      const fs = require('fs') as typeof import('fs')
+      const path = require('path') as typeof import('path')
+      const tokenPath = path.join(app.getPath('userData'), 'cloud-sync.token')
+      if (!fs.existsSync(tokenPath)) return null
+      if (!safeStorage.isEncryptionAvailable()) return null
+      const encrypted = fs.readFileSync(tokenPath)
+      return safeStorage.decryptString(encrypted)
+    } catch (err) {
+      logger.warn(`Token load failed: ${err instanceof Error ? err.message : String(err)}`)
+      return null
+    }
+  }
+
+  private _clearStoredRefreshToken(): void {
+    try {
+      const fs = require('fs') as typeof import('fs')
+      const path = require('path') as typeof import('path')
+      const tokenPath = path.join(app.getPath('userData'), 'cloud-sync.token')
+      if (fs.existsSync(tokenPath)) {
+        fs.unlinkSync(tokenPath)
+      }
+    } catch {
+      // ignore
+    }
+  }
+
+  // ── EventEmitter 타입 오버라이드 ───────────────────────
+
+  override on<K extends keyof CloudSyncEvents>(event: K, listener: CloudSyncEvents[K]): this {
+    return super.on(event, listener)
+  }
+
+  override emit<K extends keyof CloudSyncEvents>(
+    event: K,
+    ...args: Parameters<CloudSyncEvents[K]>
+  ): boolean {
+    return super.emit(event, ...args)
+  }
+}
+
+// ── 싱글톤 ─────────────────────────────────────────────
+
+let instance: CloudSyncService | null = null
+
+export function getCloudSyncService(): CloudSyncService {
+  if (!instance) {
+    instance = new CloudSyncService()
+  }
+  return instance
+}
diff --git a/apps/desktop/src/main/services/ConfigService.ts b/apps/desktop/src/main/services/ConfigService.ts
index 4f6c750..05cefb1 100644
--- a/apps/desktop/src/main/services/ConfigService.ts
+++ b/apps/desktop/src/main/services/ConfigService.ts
@@ -68,6 +68,11 @@ const CONFIG_DEFAULTS: AppConfig = {
   agentModeEnabled: false,
   handsFreeEnabled: false,
   screenContextEnabled: false,
+  hfToken: '',
+  diarizationEnabled: false,
+  supabaseUrl: '',
+  supabaseAnonKey: '',
+  cloudSyncLastAt: null,
 }
 
 let store: ElectronStore<AppConfig> | null = null
diff --git a/apps/desktop/src/preload/index.ts b/apps/desktop/src/preload/index.ts
index bb69760..9d64556 100644
--- a/apps/desktop/src/preload/index.ts
+++ b/apps/desktop/src/preload/index.ts
@@ -676,6 +676,37 @@ const electronAPI = {
     delete: (params: DeleteMeetingDocTemplateParams) =>
       invoke<void>(IPC_CHANNELS.MEETING_DOC_TEMPLATE.DELETE, params),
   },
+
+  cloudSync: {
+    getState: () =>
+      invoke<{
+        authenticated: boolean
+        userEmail: string | null
+        lastSyncAt: number | null
+        syncing: boolean
+      }>(IPC_CHANNELS.CLOUD_SYNC.GET_STATE),
+    configure: (params: { url: string; anonKey: string }) =>
+      invoke<unknown>(IPC_CHANNELS.CLOUD_SYNC.CONFIGURE, params),
+    signIn: (params: { provider: 'google' | 'github' }) =>
+      invoke<{ started: boolean }>(IPC_CHANNELS.CLOUD_SYNC.SIGN_IN, params),
+    signOut: () => invoke<unknown>(IPC_CHANNELS.CLOUD_SYNC.SIGN_OUT),
+    pushAll: () =>
+      invoke<{ pushed: number; errors: string[] }>(IPC_CHANNELS.CLOUD_SYNC.PUSH_ALL),
+    onAuthChanged: (callback: (data: { user: { id: string; email: string | null } | null }) => void) =>
+      on<{ user: { id: string; email: string | null } | null }>(
+        IPC_CHANNELS.CLOUD_SYNC.AUTH_CHANGED,
+        callback
+      ),
+    onSyncProgress: (callback: (data: { current: number; total: number; table: string }) => void) =>
+      on<{ current: number; total: number; table: string }>(
+        IPC_CHANNELS.CLOUD_SYNC.SYNC_PROGRESS,
+        callback
+      ),
+    onSyncComplete: (callback: (data: { pushed: number; errors: string[] }) => void) =>
+      on<{ pushed: number; errors: string[] }>(IPC_CHANNELS.CLOUD_SYNC.SYNC_COMPLETE, callback),
+    onSyncError: (callback: (data: { error: string }) => void) =>
+      on<{ error: string }>(IPC_CHANNELS.CLOUD_SYNC.SYNC_ERROR, callback),
+  },
 } as const
 
 contextBridge.exposeInMainWorld('electronAPI', electronAPI)
diff --git a/apps/desktop/src/renderer/components/CloudSyncSection.tsx b/apps/desktop/src/renderer/components/CloudSyncSection.tsx
new file mode 100644
index 0000000..7ee9b7f
--- /dev/null
+++ b/apps/desktop/src/renderer/components/CloudSyncSection.tsx
@@ -0,0 +1,263 @@
+// src/renderer/components/CloudSyncSection.tsx
+// Phase V2-4: Settings에 표시되는 Cloud Sync 섹션
+// 로그인 상태 / Sync Now 버튼 / 마지막 동기화 / 환경변수 설정
+
+import { useEffect, useState } from 'react'
+import { Box, Button, Stack, TextField, Alert, CircularProgress } from '@mui/material'
+import CloudIcon from '@mui/icons-material/Cloud'
+import CloudDoneIcon from '@mui/icons-material/CloudDone'
+import GoogleIcon from '@mui/icons-material/Google'
+import GitHubIcon from '@mui/icons-material/GitHub'
+import { d3roPalette, typoSx } from '@d3ro/ui/theme'
+import { useI18n } from '@d3ro/i18n'
+
+interface CloudSyncState {
+  authenticated: boolean
+  userEmail: string | null
+  lastSyncAt: number | null
+  syncing: boolean
+}
+
+interface SyncProgress {
+  current: number
+  total: number
+  table: string
+}
+
+export function CloudSyncSection(): React.ReactElement {
+  const { t: _t } = useI18n()
+  const [state, setState] = useState<CloudSyncState>({
+    authenticated: false,
+    userEmail: null,
+    lastSyncAt: null,
+    syncing: false
+  })
+  const [supabaseUrl, setSupabaseUrl] = useState('')
+  const [anonKey, setAnonKey] = useState('')
+  const [progress, setProgress] = useState<SyncProgress | null>(null)
+  const [error, setError] = useState<string | null>(null)
+  const [info, setInfo] = useState<string | null>(null)
+  const [busy, setBusy] = useState(false)
+
+  // 초기 상태 로드 + 이벤트 구독
+  useEffect(() => {
+    void window.electronAPI.cloudSync.getState().then((r) => {
+      if (r.success) setState(r.data)
+    })
+    // 저장된 supabase 설정 로드
+    void window.electronAPI.config.get({ key: 'supabaseUrl' as never }).then((r) => {
+      if (r.success && typeof r.data === 'string') setSupabaseUrl(r.data)
+    })
+    void window.electronAPI.config.get({ key: 'supabaseAnonKey' as never }).then((r) => {
+      if (r.success && typeof r.data === 'string') setAnonKey(r.data)
+    })
+
+    const unsubAuth = window.electronAPI.cloudSync.onAuthChanged((payload) => {
+      setState((prev) => ({
+        ...prev,
+        authenticated: payload.user !== null,
+        userEmail: payload.user?.email ?? null
+      }))
+    })
+    const unsubProgress = window.electronAPI.cloudSync.onSyncProgress((p) => {
+      setProgress(p)
+    })
+    const unsubComplete = window.electronAPI.cloudSync.onSyncComplete((p) => {
+      setProgress(null)
+      setInfo(`동기화 완료: ${p.pushed}건${p.errors.length > 0 ? ` (오류 ${p.errors.length})` : ''}`)
+      setBusy(false)
+      setState((prev) => ({ ...prev, syncing: false, lastSyncAt: Date.now() }))
+    })
+    const unsubError = window.electronAPI.cloudSync.onSyncError((p) => {
+      setProgress(null)
+      setError(p.error)
+      setBusy(false)
+      setState((prev) => ({ ...prev, syncing: false }))
+    })
+    return () => {
+      unsubAuth()
+      unsubProgress()
+      unsubComplete()
+      unsubError()
+    }
+  }, [])
+
+  async function handleConfigure(): Promise<void> {
+    setError(null)
+    setBusy(true)
+    try {
+      const r = await window.electronAPI.cloudSync.configure({ url: supabaseUrl, anonKey })
+      if (!r.success) {
+        setError(r.error.message)
+      } else {
+        setInfo('Supabase 연결 설정 저장됨')
+      }
+    } finally {
+      setBusy(false)
+    }
+  }
+
+  async function handleSignIn(provider: 'google' | 'github'): Promise<void> {
+    setError(null)
+    setBusy(true)
+    try {
+      const r = await window.electronAPI.cloudSync.signIn({ provider })
+      if (!r.success) {
+        setError(r.error.message)
+      } else {
+        setInfo('브라우저에서 로그인을 완료해주세요...')
+      }
+    } finally {
+      setBusy(false)
+    }
+  }
+
+  async function handleSignOut(): Promise<void> {
+    setBusy(true)
+    try {
+      await window.electronAPI.cloudSync.signOut()
+      setInfo('로그아웃되었습니다')
+    } finally {
+      setBusy(false)
+    }
+  }
+
+  async function handleSync(): Promise<void> {
+    setError(null)
+    setBusy(true)
+    setState((prev) => ({ ...prev, syncing: true }))
+    try {
+      await window.electronAPI.cloudSync.pushAll()
+    } finally {
+      // 완료/에러 이벤트로 setBusy(false) 처리됨
+    }
+  }
+
+  const lastSyncText = state.lastSyncAt
+    ? new Date(state.lastSyncAt).toLocaleString('ko-KR')
+    : '없음'
+
+  return (
+    <Box sx={{ p: 3, bgcolor: d3roPalette.bg.elevated, borderRadius: 2 }}>
+      <Stack direction="row" alignItems="center" spacing={1.5} sx={{ mb: 2 }}>
+        {state.authenticated ? (
+          <CloudDoneIcon sx={{ color: d3roPalette.tag.green }} />
+        ) : (
+          <CloudIcon sx={{ color: d3roPalette.text.label }} />
+        )}
+        <Box sx={{ ...typoSx('heading'), color: d3roPalette.text.primary }}>Cloud Sync</Box>
+      </Stack>
+
+      {!state.authenticated && (
+        <Stack spacing={2}>
+          <Box sx={{ ...typoSx('label'), color: d3roPalette.text.label }}>
+            SUPABASE 설정 (env 미설정 시 직접 입력)
+          </Box>
+          <TextField
+            label="Supabase URL"
+            size="small"
+            value={supabaseUrl}
+            onChange={(e) => setSupabaseUrl(e.target.value)}
+            placeholder="https://your-project.supabase.co"
+            fullWidth
+          />
+          <TextField
+            label="Anon Key"
+            size="small"
+            value={anonKey}
+            onChange={(e) => setAnonKey(e.target.value)}
+            placeholder="eyJ..."
+            type="password"
+            fullWidth
+          />
+          <Button variant="outlined" onClick={() => void handleConfigure()} disabled={busy}>
+            저장
+          </Button>
+
+          <Box sx={{ borderTop: `1px solid ${d3roPalette.border.subtle}`, pt: 2 }}>
+            <Box sx={{ ...typoSx('label'), color: d3roPalette.text.label, mb: 1 }}>OAuth 로그인</Box>
+            <Stack direction="row" spacing={1}>
+              <Button
+                variant="contained"
+                startIcon={<GoogleIcon />}
+                onClick={() => void handleSignIn('google')}
+                disabled={busy || !supabaseUrl || !anonKey}
+              >
+                Google
+              </Button>
+              <Button
+                variant="outlined"
+                startIcon={<GitHubIcon />}
+                onClick={() => void handleSignIn('github')}
+                disabled={busy || !supabaseUrl || !anonKey}
+              >
+                GitHub
+              </Button>
+            </Stack>
+          </Box>
+        </Stack>
+      )}
+
+      {state.authenticated && (
+        <Stack spacing={2}>
+          <Box sx={{ color: d3roPalette.text.primary, fontSize: 13 }}>
+            로그인됨: <strong>{state.userEmail ?? '(이메일 없음)'}</strong>
+          </Box>
+          <Box sx={{ color: d3roPalette.text.muted, fontSize: 12 }}>
+            마지막 동기화: {lastSyncText}
+          </Box>
+
+          {progress && (
+            <Box>
+              <Box sx={{ color: d3roPalette.text.label, fontSize: 12, mb: 0.5 }}>
+                {progress.table}: {progress.current}/{progress.total}
+              </Box>
+              <Box
+                sx={{
+                  height: 4,
+                  bgcolor: d3roPalette.bg.inset,
+                  borderRadius: 2,
+                  overflow: 'hidden'
+                }}
+              >
+                <Box
+                  sx={{
+                    height: '100%',
+                    width: `${progress.total > 0 ? (progress.current / progress.total) * 100 : 0}%`,
+                    bgcolor: d3roPalette.accent.amber,
+                    transition: 'width 200ms'
+                  }}
+                />
+              </Box>
+            </Box>
+          )}
+
+          <Stack direction="row" spacing={1}>
+            <Button
+              variant="contained"
+              onClick={() => void handleSync()}
+              disabled={busy || state.syncing}
+              startIcon={state.syncing ? <CircularProgress size={16} /> : null}
+            >
+              {state.syncing ? '동기화 중...' : 'Sync Now'}
+            </Button>
+            <Button variant="outlined" onClick={() => void handleSignOut()} disabled={busy}>
+              로그아웃
+            </Button>
+          </Stack>
+        </Stack>
+      )}
+
+      {error && (
+        <Alert severity="error" sx={{ mt: 2 }} variant="outlined">
+          {error}
+        </Alert>
+      )}
+      {info && !error && (
+        <Alert severity="info" sx={{ mt: 2 }} variant="outlined">
+          {info}
+        </Alert>
+      )}
+    </Box>
+  )
+}
diff --git a/docs/v2/phase-V2-4.md b/docs/v2/phase-V2-4.md
new file mode 100644
index 0000000..57789fa
--- /dev/null
+++ b/docs/v2/phase-V2-4.md
@@ -0,0 +1,97 @@
+# Phase V2-4: 데스크톱 ↔ Supabase 동기화 — 설계
+
+> apps/desktop이 V2-2 Supabase 인프라에 연결되어 사용자 데이터를 클라우드와 동기화한다.
+> Local-first 원칙: SQLite가 source of truth, Supabase는 미러.
+
+---
+
+## 1. 목표
+
+1. **OAuth 로그인**: 외부 브라우저로 Supabase Auth 진행 → Electron deep link 콜백
+2. **세션 영속화**: refresh token을 `electron-store`에 (가급적 `safeStorage`로 암호화) 저장
+3. **수동 동기화**: 사용자가 Settings에서 "Sync Now" 클릭 → SQLite 데이터를 Postgres로 push
+4. **자동 동기화**: 향후 기능 (V2-4b, 백그라운드 schedule)
+5. **충돌 해결**: MVP에서는 last-write-wins (created_at 기준)
+
+## 2. 동기화 매핑
+
+| 로컬 (SQLite) | 원격 (Postgres) | 변환 |
+|---|---|---|
+| `history.id` (nanoid) | `history.id` (uuid) | nanoid는 그대로 text 컬럼? → V2-4 MVP는 uuid로 새로 발급. 매핑 테이블 `sync_mapping` 필요 |
+| `history.created_at` (epoch ms int) | `history.created_at` (timestamptz) | `new Date(epochMs).toISOString()` |
+| `meeting_sessions.id` | `meetings.id` | 동일 처리 |
+| `meeting_memos.session_id` | `meeting_memos.meeting_id` | 매핑된 새 uuid |
+| `dictionary.id` | `dictionary.id` | 동일 |
+
+**매핑 테이블** (`sync_mapping`):
+```
+local_id  TEXT
+remote_id TEXT
+table     TEXT
+synced_at INTEGER
+PRIMARY KEY (local_id, table)
+```
+
+V2-4 MVP는 매핑 없이 **새 데이터만 push** (last_sync_at 이후 변경된 행). 기존 데이터는 push 안 함. 단순화.
+
+## 3. CloudSyncService 인터페이스
+
+```typescript
+class CloudSyncService extends EventEmitter {
+  // 초기화 (저장된 세션 복원)
+  async init(): Promise<void>
+
+  // 인증 상태
+  isAuthenticated(): boolean
+  getUser(): { id: string; email: string | null } | null
+
+  // OAuth 로그인 시작 (외부 브라우저 열기 + URL 반환)
+  async startSignIn(provider: 'google' | 'github'): Promise<{ authUrl: string }>
+
+  // Deep link callback 처리 (?code=...)
+  async handleAuthCallback(code: string): Promise<void>
+
+  // 로그아웃
+  async signOut(): Promise<void>
+
+  // 동기화: 마지막 sync 이후 변경된 history/meetings/dictionary push
+  async pushAll(): Promise<{ pushed: number; errors: string[] }>
+
+  // 마지막 동기화 시각
+  getLastSyncAt(): Date | null
+
+  // 이벤트:
+  //   'auth-changed' { user: User | null }
+  //   'sync-progress' { current: number; total: number; table: string }
+  //   'sync-complete' { pushed: number; errors: string[] }
+  //   'sync-error' { error: string }
+}
+```
+
+## 4. Deep link 처리
+
+- macOS: `app.setAsDefaultProtocolClient('d3ro-voice')`
+- Windows: 같은 함수 + 레지스트리
+- Linux: .desktop 파일
+
+OAuth redirect_to: `d3ro-voice://auth-callback`
+
+`app.on('open-url', ...)`: macOS 핸들러
+`app.on('second-instance', ...)`: Windows에서 두 번째 인스턴스가 deep link로 호출됐을 때
+
+## 5. UI
+
+`SettingsModal`에 새 섹션:
+- 로그아웃 상태: "Cloud Sync 로그인" 버튼 → CloudSyncService.startSignIn()
+- 로그인 상태: 사용자 이메일 + "Sync Now" 버튼 + "마지막 동기화: ..." + "로그아웃"
+
+## 6. 검증
+
+- typecheck + build
+- dev 실행 → Settings에서 로그인 버튼 나타남
+- 실제 동기화는 Supabase 배포 후 (사용자 액션)
+
+## 7. V2-4 → V2-4b 분리
+
+- V2-4 (이번): push만, 수동 트리거
+- V2-4b (나중): pull + 양방향 + Realtime 구독 + 자동 schedule
diff --git a/package-lock.json b/package-lock.json
index dde989b..f236496 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -26,6 +26,7 @@
       "version": "1.0.0",
       "license": "MIT",
       "dependencies": {
+        "@d3ro/api-client": "*",
         "@d3ro/core": "*",
         "@d3ro/i18n": "*",
         "@d3ro/ui": "*",
@@ -38,6 +39,7 @@
         "@mui/material": "^7.0.0",
         "@nut-tree-fork/nut-js": "^4.2.6",
         "@rollup/rollup-win32-x64-msvc": "^4.60.1",
+        "@supabase/supabase-js": "^2.45.0",
         "better-sqlite3": "^12.8.0",
         "docx": "^9.6.1",
         "drizzle-orm": "^0.45.2",
diff --git a/packages/core/src/ipc-channels.ts b/packages/core/src/ipc-channels.ts
index f2f7a6e..2b2ef9c 100644
--- a/packages/core/src/ipc-channels.ts
+++ b/packages/core/src/ipc-channels.ts
@@ -369,6 +369,20 @@ export const IPC_CHANNELS = {
     UPGRADE_PROMPT: 'license:upgradePrompt',
     TIER_CHANGED: 'license:tierChanged',
   },
+
+  CLOUD_SYNC: {
+    GET_STATE: 'cloudSync:getState',
+    SIGN_IN: 'cloudSync:signIn',
+    SIGN_OUT: 'cloudSync:signOut',
+    PUSH_ALL: 'cloudSync:pushAll',
+    HANDLE_CALLBACK: 'cloudSync:handleCallback',
+    CONFIGURE: 'cloudSync:configure',
+    // events
+    AUTH_CHANGED: 'cloudSync:authChanged',
+    SYNC_PROGRESS: 'cloudSync:syncProgress',
+    SYNC_COMPLETE: 'cloudSync:syncComplete',
+    SYNC_ERROR: 'cloudSync:syncError',
+  },
 } as const
 
 // 타입 유틸리티: 채널명 유니온 추출
diff --git a/packages/core/src/types.ts b/packages/core/src/types.ts
index 0ddd430..32b95f8 100644
--- a/packages/core/src/types.ts
+++ b/packages/core/src/types.ts
@@ -388,6 +388,12 @@ export interface AppConfig {
   hfToken: string
   /** Phase 15.5: 화자 구분 활성화 */
   diarizationEnabled: boolean
+  /** V2-4: Supabase 프로젝트 URL */
+  supabaseUrl: string
+  /** V2-4: Supabase anon public key */
+  supabaseAnonKey: string
+  /** V2-4: 마지막 동기화 epoch ms */
+  cloudSyncLastAt: number | null
 }
 
 export interface ConfigGetParams {