diff --git a/__tests__/lib/highlight-sync-manager.test.ts b/__tests__/lib/highlight-sync-manager.test.ts
new file mode 100644
index 0000000..a5093eb
--- /dev/null
+++ b/__tests__/lib/highlight-sync-manager.test.ts
@@ -0,0 +1,79 @@
+import { HighlightSyncManager } from '@/lib/highlight-sync-manager'
+import { BibleHighlight } from '@/types'
+
+describe('HighlightSyncManager', () => {
+  let manager: HighlightSyncManager
+
+  beforeEach(() => {
+    manager = new HighlightSyncManager()
+  })
+
+  it('should add highlight to sync queue', async () => {
+    const highlight: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: Date.now(),
+      updatedAt: Date.now(),
+      syncStatus: 'pending'
+    }
+
+    await manager.queueHighlight(highlight)
+    const pending = await manager.getPendingSyncItems()
+
+    expect(pending.length).toBe(1)
+    expect(pending[0].id).toBe('h-1')
+  })
+
+  it('should mark highlight as syncing', async () => {
+    const highlight: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: Date.now(),
+      updatedAt: Date.now(),
+      syncStatus: 'pending'
+    }
+
+    await manager.queueHighlight(highlight)
+    await manager.markSyncing(['h-1'])
+
+    const syncing = await manager.getSyncingItems()
+    expect(syncing.length).toBe(1)
+  })
+
+  it('should mark highlight as synced', async () => {
+    const highlight: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: Date.now(),
+      updatedAt: Date.now(),
+      syncStatus: 'pending'
+    }
+
+    await manager.queueHighlight(highlight)
+    await manager.markSynced(['h-1'])
+
+    const pending = await manager.getPendingSyncItems()
+    expect(pending.length).toBe(0)
+  })
+
+  it('should retry sync on error', async () => {
+    const highlight: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: Date.now(),
+      updatedAt: Date.now(),
+      syncStatus: 'pending'
+    }
+
+    await manager.queueHighlight(highlight)
+    await manager.markError(['h-1'], 'Network error')
+    await manager.markSyncing(['h-1'])
+
+    const syncing = await manager.getSyncingItems()
+    expect(syncing.length).toBe(1)
+  })
+})
diff --git a/lib/highlight-sync-manager.ts b/lib/highlight-sync-manager.ts
new file mode 100644
index 0000000..7f628e0
--- /dev/null
+++ b/lib/highlight-sync-manager.ts
@@ -0,0 +1,126 @@
+import { BibleHighlight, HighlightSyncQueueItem } from '@/types'
+import {
+  initHighlightsDatabase,
+  updateHighlight,
+  getHighlight
+} from './highlight-manager'
+
+const SYNC_QUEUE_STORE = 'highlight_sync_queue'
+
+export class HighlightSyncManager {
+  private db: IDBDatabase | null = null
+  private syncInterval: NodeJS.Timeout | null = null
+
+  async init() {
+    this.db = await initHighlightsDatabase()
+
+    // Create sync queue store if it doesn't exist
+    if (!this.db.objectStoreNames.contains(SYNC_QUEUE_STORE)) {
+      // Note: In real app, this would be done in onupgradeneeded
+      // For this implementation, assume schema is managed separately
+    }
+  }
+
+  async queueHighlight(highlight: BibleHighlight): Promise<void> {
+    if (!this.db) await this.init()
+
+    const queueItem: HighlightSyncQueueItem = {
+      highlightId: highlight.id,
+      action: highlight.syncStatus === 'synced' ? 'update' : 'create',
+      highlight,
+      retryCount: 0
+    }
+
+    await updateHighlight({
+      ...highlight,
+      syncStatus: 'pending'
+    })
+  }
+
+  async getPendingSyncItems(): Promise<BibleHighlight[]> {
+    if (!this.db) await this.init()
+
+    return new Promise((resolve, reject) => {
+      const tx = this.db!.transaction('highlights', 'readonly')
+      const store = tx.objectStore('highlights')
+      const index = store.index('syncStatus')
+      const request = index.getAll('pending')
+
+      request.onsuccess = () => resolve(request.result || [])
+      request.onerror = () => reject(new Error('Failed to get pending items'))
+    })
+  }
+
+  async getSyncingItems(): Promise<BibleHighlight[]> {
+    if (!this.db) await this.init()
+
+    return new Promise((resolve, reject) => {
+      const tx = this.db!.transaction('highlights', 'readonly')
+      const store = tx.objectStore('highlights')
+      const index = store.index('syncStatus')
+      const request = index.getAll('syncing')
+
+      request.onsuccess = () => resolve(request.result || [])
+      request.onerror = () => reject(new Error('Failed to get syncing items'))
+    })
+  }
+
+  async markSyncing(highlightIds: string[]): Promise<void> {
+    if (!this.db) await this.init()
+
+    for (const id of highlightIds) {
+      const highlight = await getHighlight(id)
+      if (highlight) {
+        await updateHighlight({
+          ...highlight,
+          syncStatus: 'syncing'
+        })
+      }
+    }
+  }
+
+  async markSynced(highlightIds: string[]): Promise<void> {
+    if (!this.db) await this.init()
+
+    for (const id of highlightIds) {
+      const highlight = await getHighlight(id)
+      if (highlight) {
+        await updateHighlight({
+          ...highlight,
+          syncStatus: 'synced'
+        })
+      }
+    }
+  }
+
+  async markError(highlightIds: string[], errorMsg: string): Promise<void> {
+    if (!this.db) await this.init()
+
+    for (const id of highlightIds) {
+      const highlight = await getHighlight(id)
+      if (highlight) {
+        await updateHighlight({
+          ...highlight,
+          syncStatus: 'error',
+          syncErrorMsg: errorMsg
+        })
+      }
+    }
+  }
+
+  startAutoSync(intervalMs: number = 30000, onSyncNeeded?: () => void) {
+    this.syncInterval = setInterval(async () => {
+      const pending = await this.getPendingSyncItems()
+      if (pending.length > 0 && onSyncNeeded) {
+        onSyncNeeded()
+      }
+    }, intervalMs)
+  }
+
+  stopAutoSync() {
+    if (this.syncInterval) {
+      clearInterval(this.syncInterval)
+      this.syncInterval = null
+    }
+  }
+}