diff --git a/src/App.vue b/src/App.vue
index 5d57b0e..2a7c525 100644
--- a/src/App.vue
+++ b/src/App.vue
@@ -9,6 +9,8 @@ import LoginModal from '@/components/LoginModal.vue'
 import SignupModal from '@/components/SignupModal.vue'
 import WebOnlyNotice from '@/components/WebOnlyNotice.vue'
 import AppDialog from '@/components/ui/AppDialog.vue'
+import AppLockScreen from '@/components/AppLockScreen.vue'
+import { appLock } from '@/composables/appLock'
 import { Capacitor } from '@capacitor/core'
 import { reminders } from '@/native/reminders'
 import { useAuthStore } from '@/stores/auth'
@@ -71,6 +73,12 @@ function retryReload() {
   window.location.reload()
 }
 
+// ===== 앱 잠금(PIN) =====
+// 백그라운드로 가면 잠가서, 복귀 시 잠금 화면이 떠 있도록 한다.
+function onVisibility() {
+  if (document.hidden) appLock.lock()
+}
+
 onMounted(() => {
   if (!isApp) return // 웹은 안내만 — 앱 전용 초기화 생략
   window.addEventListener('auth:unauthorized', onUnauthorized)
@@ -78,6 +86,9 @@ onMounted(() => {
   window.addEventListener('online', onOnline)
   window.addEventListener('offline', onOffline)
   window.addEventListener('net:error', onNetError)
+  // 앱 잠금: 저장된 세션으로 재진입하는 경우 시작 시 잠금(새 로그인 직후는 잠그지 않음)
+  if (appLock.state.enabled && appLock.hasSession()) appLock.lock()
+  document.addEventListener('visibilitychange', onVisibility)
   ui.loadSignupEnabled() // 회원가입 허용 여부 선로딩(랜딩/로그인 가입 버튼에 반영)
   // 로그인 상태면 전체 프로필(아바타·포인트) 최신화 — 재로그인 없이 헤더 아바타 반영
   if (auth.isAuthenticated) {
@@ -99,6 +110,7 @@ onUnmounted(() => {
   window.removeEventListener('online', onOnline)
   window.removeEventListener('offline', onOffline)
   window.removeEventListener('net:error', onNetError)
+  document.removeEventListener('visibilitychange', onVisibility)
   clearTimeout(netTimer)
 })
 
@@ -115,6 +127,9 @@ watch(() => route.fullPath, () => ui.closeSidebar())
 
   <!-- 앱(Capacitor): 전체 기능 -->
   <template v-else>
+    <!-- 앱 잠금 화면 (로그인 상태 + 잠금 활성 시) -->
+    <AppLockScreen v-if="appLock.state.locked && auth.isAuthenticated" />
+
     <!-- 네트워크 오류/오프라인 안내 배너 -->
     <Transition name="netbar">
       <div v-if="showNetBanner" class="net-banner" role="alert">
diff --git a/src/components/AppLockScreen.vue b/src/components/AppLockScreen.vue
new file mode 100644
index 0000000..3577c0a
--- /dev/null
+++ b/src/components/AppLockScreen.vue
@@ -0,0 +1,96 @@
+<script setup>
+import { ref } from 'vue'
+import { appLock, PIN_LENGTH } from '@/composables/appLock'
+import { useAuthStore } from '@/stores/auth'
+import PinPad from '@/components/PinPad.vue'
+
+const auth = useAuthStore()
+const pin = ref('')
+const error = ref(false)
+
+async function onComplete(p) {
+  if (await appLock.verify(p)) {
+    pin.value = ''
+    error.value = false
+    appLock.unlock()
+  } else {
+    error.value = true
+    pin.value = ''
+  }
+}
+function onInput(v) {
+  pin.value = v
+  error.value = false
+}
+async function forgot() {
+  // PIN 분실 → 잠금 해제 + 로그아웃(재로그인 후 다시 설정). 갇힘 방지.
+  appLock.disable()
+  await auth.logout()
+  appLock.unlock()
+}
+</script>
+
+<template>
+  <div class="lock-screen">
+    <div class="lock-inner">
+      <div class="lock-logo">🐷</div>
+      <h1 class="lock-title">돈돼지 가계부</h1>
+      <p class="lock-sub" :class="{ err: error }">
+        {{ error ? 'PIN이 일치하지 않아요' : 'PIN을 입력하세요' }}
+      </p>
+      <PinPad :value="pin" :length="PIN_LENGTH" class="lock-pad" @update:value="onInput" @complete="onComplete" />
+      <button type="button" class="lock-forgot" @click="forgot">PIN을 잊으셨나요? 로그아웃</button>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.lock-screen {
+  position: fixed;
+  inset: 0;
+  z-index: 3000;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: calc(env(safe-area-inset-top, 0) + 1rem) 1rem calc(env(safe-area-inset-bottom, 0) + 1rem);
+  background: var(--color-background);
+}
+.lock-inner {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  text-align: center;
+}
+.lock-logo {
+  font-size: 2.6rem;
+}
+.lock-title {
+  margin-top: 0.4rem;
+  font-size: 1.25rem;
+  color: var(--color-heading);
+}
+.lock-sub {
+  margin-top: 0.6rem;
+  margin-bottom: 1.8rem;
+  font-size: 0.92rem;
+  opacity: 0.7;
+}
+.lock-sub.err {
+  color: #c0392b;
+  opacity: 1;
+  font-weight: 600;
+}
+.lock-forgot {
+  margin-top: 2rem;
+  border: 0;
+  background: transparent;
+  color: var(--color-text);
+  opacity: 0.6;
+  font-size: 0.85rem;
+  text-decoration: underline;
+  cursor: pointer;
+}
+.lock-forgot:hover {
+  opacity: 0.9;
+}
+</style>
diff --git a/src/components/PinPad.vue b/src/components/PinPad.vue
new file mode 100644
index 0000000..bd61bf4
--- /dev/null
+++ b/src/components/PinPad.vue
@@ -0,0 +1,85 @@
+<script setup>
+// 재사용 PIN 키패드 — 점 표시 + 숫자 키패드. 길이 도달 시 complete 방출.
+const props = defineProps({
+  value: { type: String, default: '' },
+  length: { type: Number, default: 4 },
+})
+const emit = defineEmits(['update:value', 'complete'])
+
+function press(d) {
+  if (props.value.length >= props.length) return
+  const next = props.value + d
+  emit('update:value', next)
+  if (next.length === props.length) emit('complete', next)
+}
+function del() {
+  emit('update:value', props.value.slice(0, -1))
+}
+</script>
+
+<template>
+  <div class="pinpad">
+    <div class="dots">
+      <span v-for="i in length" :key="i" class="dot" :class="{ on: value.length >= i }"></span>
+    </div>
+    <div class="keys">
+      <button v-for="n in 9" :key="n" type="button" class="key" @click="press(String(n))">{{ n }}</button>
+      <span class="key empty" aria-hidden="true"></span>
+      <button type="button" class="key" @click="press('0')">0</button>
+      <button type="button" class="key del" aria-label="지우기" @click="del">⌫</button>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.pinpad {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 1.6rem;
+}
+.dots {
+  display: flex;
+  gap: 0.9rem;
+}
+.dot {
+  width: 14px;
+  height: 14px;
+  border-radius: 50%;
+  border: 2px solid currentColor;
+  opacity: 0.5;
+}
+.dot.on {
+  background: currentColor;
+  opacity: 1;
+}
+.keys {
+  display: grid;
+  grid-template-columns: repeat(3, 64px);
+  gap: 0.8rem;
+}
+.key {
+  width: 64px;
+  height: 64px;
+  border-radius: 50%;
+  border: 1px solid var(--color-border);
+  background: var(--color-background-soft);
+  color: var(--color-text);
+  font-size: 1.4rem;
+  font-weight: 600;
+  cursor: pointer;
+}
+.key:active {
+  background: var(--color-border);
+}
+.key.empty {
+  border: 0;
+  background: transparent;
+  cursor: default;
+}
+.key.del {
+  font-size: 1.2rem;
+  border: 0;
+  background: transparent;
+}
+</style>
diff --git a/src/composables/appLock.js b/src/composables/appLock.js
new file mode 100644
index 0000000..309780c
--- /dev/null
+++ b/src/composables/appLock.js
@@ -0,0 +1,54 @@
+import { reactive } from 'vue'
+
+// 앱 잠금(PIN). 자체 구현 — 네이티브 플러그인 없이 동작.
+// PIN 은 평문 저장하지 않고 SHA-256 해시만 보관. 기본 비활성(옵트인).
+// 갇힘 방지: 잠금 화면에서 'PIN 분실 → 로그아웃' 으로 disable + 로그아웃 가능.
+const KEY_ENABLED = 'applock:enabled'
+const KEY_HASH = 'applock:hash'
+
+export const PIN_LENGTH = 4
+
+const state = reactive({
+  enabled: localStorage.getItem(KEY_ENABLED) === '1',
+  locked: false,
+})
+
+async function hashPin(pin) {
+  const data = new TextEncoder().encode('dondwaeji-applock:' + pin)
+  const buf = await crypto.subtle.digest('SHA-256', data)
+  return Array.from(new Uint8Array(buf)).map((b) => b.toString(16).padStart(2, '0')).join('')
+}
+
+export const appLock = {
+  state,
+  /** PIN 설정 + 잠금 활성화 */
+  async setPin(pin) {
+    localStorage.setItem(KEY_HASH, await hashPin(pin))
+    localStorage.setItem(KEY_ENABLED, '1')
+    state.enabled = true
+  },
+  /** 입력 PIN 검증 */
+  async verify(pin) {
+    const stored = localStorage.getItem(KEY_HASH)
+    return !!stored && (await hashPin(pin)) === stored
+  },
+  /** 잠금 해제(설정 끄기) */
+  disable() {
+    localStorage.removeItem(KEY_HASH)
+    localStorage.removeItem(KEY_ENABLED)
+    state.enabled = false
+    state.locked = false
+  },
+  /** 잠금 걸기(활성화된 경우에만) */
+  lock() {
+    if (state.enabled) state.locked = true
+  },
+  /** 잠금 풀기(이번 세션) */
+  unlock() {
+    state.locked = false
+  },
+  /** 저장된 세션이 있는지(앱 시작 시 잠금 여부 판단용) */
+  hasSession() {
+    return !!localStorage.getItem('token')
+  },
+}
diff --git a/src/views/settings/SettingsView.vue b/src/views/settings/SettingsView.vue
index 644d769..3a2def8 100644
--- a/src/views/settings/SettingsView.vue
+++ b/src/views/settings/SettingsView.vue
@@ -7,6 +7,8 @@ import { useUiStore } from '@/stores/ui'
 import { useAuthStore } from '@/stores/auth'
 import { exportBackup, importBackup } from '@/utils/backup'
 import { reminders } from '@/native/reminders'
+import { appLock, PIN_LENGTH } from '@/composables/appLock'
+import PinPad from '@/components/PinPad.vue'
 
 const dialog = useDialog()
 const ui = useUiStore()
@@ -61,6 +63,46 @@ async function changeNotifHour(v) {
   if (notifEnabled.value) await reminders.scheduleEntryReminder(false)
 }
 
+// ===== 앱 잠금(PIN) =====
+const lockEnabled = computed(() => appLock.state.enabled)
+const lockSetupOpen = ref(false)
+const setupStep = ref(1) // 1: 입력, 2: 확인
+const setupPin = ref('')
+const setupFirst = ref('')
+const setupError = ref('')
+
+function toggleLock() {
+  if (lockEnabled.value) confirmDisableLock()
+  else openLockSetup()
+}
+function openLockSetup() {
+  setupStep.value = 1
+  setupPin.value = ''
+  setupFirst.value = ''
+  setupError.value = ''
+  lockSetupOpen.value = true
+}
+async function onSetupComplete(p) {
+  if (setupStep.value === 1) {
+    setupFirst.value = p
+    setupPin.value = ''
+    setupStep.value = 2
+    setupError.value = ''
+  } else if (p === setupFirst.value) {
+    await appLock.setPin(p)
+    lockSetupOpen.value = false
+  } else {
+    setupError.value = 'PIN이 일치하지 않아요. 처음부터 다시 설정해주세요.'
+    setupStep.value = 1
+    setupPin.value = ''
+    setupFirst.value = ''
+  }
+}
+async function confirmDisableLock() {
+  const ok = await dialog.confirm('앱 잠금을 끌까요? 다음 실행부터 PIN 없이 열립니다.', { title: '앱 잠금 해제' })
+  if (ok) appLock.disable()
+}
+
 const exporting = ref(false)
 async function doExport() {
   if (exporting.value) return
@@ -198,6 +240,21 @@ async function clearAppData() {
       </div>
     </section>
 
+    <!-- 앱 잠금 (PIN) -->
+    <section class="card">
+      <div class="row">
+        <div class="row-main">
+          <span class="row-label">앱 잠금 (PIN)</span>
+          <span class="row-sub">앱 실행·복귀 시 {{ PIN_LENGTH }}자리 PIN 입력</span>
+        </div>
+        <button
+          type="button" class="switch" :class="{ on: lockEnabled }"
+          role="switch" :aria-checked="lockEnabled"
+          @click="toggleLock"
+        ><span class="knob"></span></button>
+      </div>
+    </section>
+
     <!-- PC(데스크톱) 전용: 창 크기 (가로·세로 각각) -->
     <section v-if="isDesktop" class="card">
       <div class="row">
@@ -295,6 +352,20 @@ async function clearAppData() {
     </section>
 
     <p class="copyright">© 2026 돈돼지 가계부. All rights reserved.</p>
+
+    <!-- PIN 설정 모달 -->
+    <Teleport to="body">
+      <div v-if="lockSetupOpen" class="pin-backdrop" @click.self="lockSetupOpen = false">
+        <div class="pin-modal" role="dialog" aria-modal="true" aria-label="PIN 설정">
+          <button type="button" class="pin-close" aria-label="닫기" @click="lockSetupOpen = false">×</button>
+          <h2 class="pin-h">{{ setupStep === 1 ? 'PIN 설정' : 'PIN 다시 입력' }}</h2>
+          <p class="pin-sub" :class="{ err: setupError }">
+            {{ setupError || (setupStep === 1 ? `사용할 ${PIN_LENGTH}자리 PIN을 입력하세요` : '확인을 위해 한 번 더 입력하세요') }}
+          </p>
+          <PinPad :value="setupPin" :length="PIN_LENGTH" @update:value="(v) => { setupPin = v; setupError = '' }" @complete="onSetupComplete" />
+        </div>
+      </div>
+    </Teleport>
   </div>
 </template>
 
@@ -303,6 +374,53 @@ async function clearAppData() {
   max-width: 560px;
   margin: 0 auto;
 }
+/* PIN 설정 모달 */
+.pin-backdrop {
+  position: fixed;
+  inset: 0;
+  z-index: 2500;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 1rem;
+  background: rgba(0, 0, 0, 0.5);
+}
+.pin-modal {
+  position: relative;
+  width: 100%;
+  max-width: 340px;
+  padding: 1.6rem 1.2rem 1.8rem;
+  border-radius: 14px;
+  background: var(--color-background);
+  border: 1px solid var(--color-border);
+  text-align: center;
+}
+.pin-close {
+  position: absolute;
+  top: 0.5rem;
+  right: 0.6rem;
+  width: 30px;
+  height: 30px;
+  border: 0;
+  background: transparent;
+  color: var(--color-text);
+  font-size: 1.4rem;
+  cursor: pointer;
+}
+.pin-h {
+  font-size: 1.1rem;
+  color: var(--color-heading);
+}
+.pin-sub {
+  margin: 0.4rem 0 1.4rem;
+  font-size: 0.85rem;
+  opacity: 0.7;
+}
+.pin-sub.err {
+  color: #c0392b;
+  opacity: 1;
+  font-weight: 600;
+}
 .page-title {
   font-size: 1.4rem;
   margin-bottom: 1rem;