diff --git a/src/main/ipc/settingsApi.ts b/src/main/ipc/settingsApi.ts
index 063194b..5b1bf59 100644
--- a/src/main/ipc/settingsApi.ts
+++ b/src/main/ipc/settingsApi.ts
@@ -112,6 +112,17 @@ export function registerSettingsApi(deps?: SettingsIpcDeps): void {
     return { ok: true as const };
   });
 
+  // v0.4 — Sidebar UI state 영속화
+  ipcMain.handle('settings:set-sidebar-visible', async (_e, visible: boolean) => {
+    await settings.setSidebarVisible(visible);
+    return { ok: true as const };
+  });
+
+  ipcMain.handle('settings:set-sidebar-width', async (_e, width: number) => {
+    await settings.setSidebarWidth(width);
+    return { ok: true as const };
+  });
+
   ipcMain.handle('settings:set-sync-auto-enabled', async (_e, value: boolean) => {
     await deps.settings.setAutoSyncEnabled(value);
     await deps.syncTimer?.reconfigure();
diff --git a/src/preload/index.ts b/src/preload/index.ts
index 503e92c..1ba7c6f 100644
--- a/src/preload/index.ts
+++ b/src/preload/index.ts
@@ -81,6 +81,9 @@ const api: InklingApi = {
     getSettings: () => ipcRenderer.invoke('settings:get'),
     setAiEnabled: (enabled: boolean) => ipcRenderer.invoke('settings:set-ai-enabled', enabled),
     setOnboardingCompleted: (completed: boolean) => ipcRenderer.invoke('settings:set-onboarding-completed', completed),
+    // v0.4 — Sidebar UI state 영속화
+    setSidebarVisible: (visible: boolean) => ipcRenderer.invoke('settings:set-sidebar-visible', visible),
+    setSidebarWidth: (width: number) => ipcRenderer.invoke('settings:set-sidebar-width', width),
     // v0.2.9 Cut B Task 16 — disabled 메모 재투입 + count.
     enqueueDisabled: () => ipcRenderer.invoke('inbox:enqueue-disabled'),
     getDisabledCount: () => ipcRenderer.invoke('inbox:get-disabled-count'),
diff --git a/src/renderer/inbox/store.ts b/src/renderer/inbox/store.ts
index 0569ef8..e8ba3bd 100644
--- a/src/renderer/inbox/store.ts
+++ b/src/renderer/inbox/store.ts
@@ -152,7 +152,14 @@ export const useInbox = create<InboxState>((set, get) => ({
         inboxApi.countsByStatus({ notebookId }),
         inboxApi.getSettings()
       ]);
-      set({ notes, continuity, pendingCount, ollamaStatus, todayCount, trashCount, expiredCandidates, failedCount, recallCandidate, counts, ai_enabled: settings.ai_enabled ?? true, loading: false });
+      set({
+        notes, continuity, pendingCount, ollamaStatus, todayCount, trashCount, expiredCandidates,
+        failedCount, recallCandidate, counts,
+        ai_enabled: settings.ai_enabled ?? true,
+        sidebarVisible: settings.sidebar_visible ?? false,
+        sidebarWidth: settings.sidebar_width ?? 240,
+        loading: false
+      });
     } catch (e) {
       // 첫 launch 의 IPC 실패 (DB migration 실패 / main process 비정상) 시 무한 loading 회피.
       // 빈 데이터로 진입하면 사용자가 캡처 시도 → 실제 fail 이 표면화 → 재시도 가능.
@@ -480,7 +487,9 @@ export const useInbox = create<InboxState>((set, get) => ({
     await get().refreshMeta();
   },
   toggleSidebar() {
-    set({ sidebarVisible: !get().sidebarVisible });
+    const next = !get().sidebarVisible;
+    set({ sidebarVisible: next });
+    void inboxApi.setSidebarVisible(next);
   },
   // v0.4 Task 11 — promotion candidate actions.
   async loadPromotionCandidates() {
diff --git a/src/shared/types.ts b/src/shared/types.ts
index 2fb75b1..6e50f9f 100644
--- a/src/shared/types.ts
+++ b/src/shared/types.ts
@@ -228,8 +228,13 @@ export interface InboxApi {
     vision_model?: string | null;
     vision_capable_cache?: string[];
     vision_cache_at?: string;
+    // v0.4 — Sidebar UI state 영속화
+    sidebar_visible?: boolean;
+    sidebar_width?: number;
   }>;
   setAiEnabled(enabled: boolean): Promise<{ ok: true }>;
+  setSidebarVisible(visible: boolean): Promise<{ ok: true }>;
+  setSidebarWidth(width: number): Promise<{ ok: true }>;
   setOnboardingCompleted(completed: boolean): Promise<{ ok: true }>;
   // v0.2.9 Cut B Task 16 — ai_status='disabled' 메모 재투입 (사용자가 ai_enabled OFF→ON 전환 시).
   enqueueDisabled(): Promise<{ count: number }>;
diff --git a/tests/unit/App.test.tsx b/tests/unit/App.test.tsx
index 0111a18..c96d529 100644
--- a/tests/unit/App.test.tsx
+++ b/tests/unit/App.test.tsx
@@ -56,6 +56,8 @@ vi.mock('../../src/renderer/inbox/api.js', () => ({
     // v0.2.9 Cut B Task 12 — onboarding wizard 분기. default 는 onboarding_completed=true 라 wizard 미표시.
     getSettings: vi.fn(async () => ({ onboarding_completed: true })),
     setAiEnabled: vi.fn(async () => ({ ok: true as const })),
+    setSidebarVisible: vi.fn(async () => ({ ok: true as const })),
+    setSidebarWidth: vi.fn(async () => ({ ok: true as const })),
     setOnboardingCompleted: vi.fn(async () => ({ ok: true as const })),
     // v0.2.9 Cut B Task 16 — AiProviderSection 가 SettingsPage 렌더 시 호출.
     getDisabledCount: vi.fn(async () => 0),
@@ -174,11 +176,12 @@ describe('App header — 3 tabs (v0.4)', () => {
   });
 
   it('Sidebar 컴포넌트가 렌더 트리에 포함됨 (sidebarVisible=true)', async () => {
-    useInbox.setState({ sidebarVisible: true, notebooks: [] });
+    // loadInitial 의 getSettings 가 sidebar_visible=true 반환 (Strict Mode 중복 호출 대비 mockResolvedValue).
+    vi.mocked(inboxApi.getSettings).mockResolvedValue({ onboarding_completed: true, sidebar_visible: true });
     render(<App />);
     await screen.findByRole('tab', { name: /Inbox/ });
-    // Sidebar renders an <aside> element when visible
-    expect(document.querySelector('aside')).not.toBeNull();
+    // loadInitial 비동기 hydrate 가 완료될 때까지 기다림
+    await waitFor(() => expect(document.querySelector('aside')).not.toBeNull());
   });
 });
 
diff --git a/tests/unit/store.notebook.test.ts b/tests/unit/store.notebook.test.ts
index 16e312a..e9b007d 100644
--- a/tests/unit/store.notebook.test.ts
+++ b/tests/unit/store.notebook.test.ts
@@ -19,7 +19,9 @@ vi.mock('../../src/renderer/inbox/api.js', () => ({
     listRecallCandidate: vi.fn(async () => null),
     countsByStatus: mockCountsByStatus,
     getSettings: vi.fn(async () => ({ ai_enabled: true })),
-    listByStatus: mockListByStatus
+    listByStatus: mockListByStatus,
+    setSidebarVisible: vi.fn(async () => ({ ok: true as const })),
+    setSidebarWidth: vi.fn(async () => ({ ok: true as const }))
   },
   notebookApi: {
     list: vi.fn(async () => [