diff --git a/.changeset/split-meow-mixed-currency-proactive-guard.md b/.changeset/split-meow-mixed-currency-proactive-guard.md
new file mode 100644
index 000000000..3f01214d5
--- /dev/null
+++ b/.changeset/split-meow-mixed-currency-proactive-guard.md
@@ -0,0 +1,5 @@
+---
+'@app/split-meow': patch
+---
+
+切換幣別或記帳若會使同一行程混用多種幣別，改在當下以確認對話框提示；編輯舊費用時改以行程主導幣別解析符號，避免顯示錯誤幣別。時區自動偵測幣別若會造成混幣則靜默跳過，不再覆寫使用者幣別。
diff --git a/apps/split-meow/src/components/Calculator.tsx b/apps/split-meow/src/components/Calculator.tsx
index 7c53222f4..09713dfd1 100644
--- a/apps/split-meow/src/components/Calculator.tsx
+++ b/apps/split-meow/src/components/Calculator.tsx
@@ -2,6 +2,7 @@ import { useTranslation } from 'react-i18next';
 import { useStore } from '../store/useStore';
 import { cn } from '../lib/utils';
 import { evaluateExpression } from '../lib/evaluateExpression';
+import { confirmMixedCurrencyIfNeeded } from '../config/currencies';
 
 interface CalculatorProps {
   onPawParticle?: (x: number, y: number) => void;
@@ -17,6 +18,9 @@ export function Calculator({ onPawParticle }: CalculatorProps = {}) {
     setCalculatorValue,
     setItemizedValue,
     saveExpense,
+    expenses,
+    currentTripId,
+    currency,
   } = useStore();
 
   const currentValue =
@@ -31,6 +35,21 @@ export function Calculator({ onPawParticle }: CalculatorProps = {}) {
       ? evaluateExpression(currentValue) > 0
       : Object.values(itemizedValues).some((v) => evaluateExpression(v) > 0);
 
+  const handleSave = () => {
+    const tripExpenses = expenses.filter((e) => e.tripId === currentTripId);
+    if (
+      !confirmMixedCurrencyIfNeeded(
+        tripExpenses,
+        currency,
+        currency,
+        t('history.mixed_currency_confirm'),
+      )
+    ) {
+      return;
+    }
+    saveExpense();
+  };
+
   const handlePress = (key: string) => {
     if (splitMode === 'itemized' && !focusedMemberId) return;
     if ('vibrate' in navigator) navigator.vibrate(8);
@@ -191,7 +210,7 @@ export function Calculator({ onPawParticle }: CalculatorProps = {}) {
         </button>
       ))}
       <button
-        onClick={canSave ? saveExpense : undefined}
+        onClick={canSave ? handleSave : undefined}
         disabled={!canSave}
         className={cn(
           'relative overflow-hidden rounded-full transition-all flex items-center justify-center gap-2 shadow-ambient text-lg sm:text-xl font-medium',
diff --git a/apps/split-meow/src/components/HistoryTab.tsx b/apps/split-meow/src/components/HistoryTab.tsx
index cb105d205..bd7d03c9e 100644
--- a/apps/split-meow/src/components/HistoryTab.tsx
+++ b/apps/split-meow/src/components/HistoryTab.tsx
@@ -924,7 +924,9 @@ interface EditExpenseSheetProps {
 function EditExpenseSheet({ expense, members, onSave, onClose }: EditExpenseSheetProps) {
   const { t } = useTranslation();
   const globalCurrency = useStore((s) => s.currency);
-  const expenseCurrency = expense.currency ?? globalCurrency;
+  const tripExpenses = useStore((s) => s.expenses.filter((e) => e.tripId === s.currentTripId));
+  const tripCurrency = resolveTripCurrency(tripExpenses, globalCurrency);
+  const expenseCurrency = resolveExpenseCurrency(expense, tripCurrency);
   const isEvenly = expense.type === 'split_evenly';
 
   const [totalInput, setTotalInput] = useState(
diff --git a/apps/split-meow/src/components/SettingsTab.tsx b/apps/split-meow/src/components/SettingsTab.tsx
index 910999d50..74a5808a4 100644
--- a/apps/split-meow/src/components/SettingsTab.tsx
+++ b/apps/split-meow/src/components/SettingsTab.tsx
@@ -5,7 +5,7 @@ import { MemberAvatar } from './MemberAvatar';
 import i18n, { type SupportedLanguage } from '../i18n';
 import { getDisplayVersion } from '../config/version';
 import { cn } from '../lib/utils';
-import { CURRENCIES, type CurrencyCode } from '../config/currencies';
+import { CURRENCIES, type CurrencyCode, confirmMixedCurrencyIfNeeded } from '../config/currencies';
 
 const LANGUAGES: { id: SupportedLanguage; flag: string; name: string }[] = [
   { id: 'zh-TW', flag: '🇹🇼', name: '繁中' },
@@ -28,6 +28,8 @@ export function SettingsTab() {
     currencyManuallySet,
     setCurrency,
     rateUpdatedAt,
+    expenses,
+    currentTripId,
   } = useStore();
   const me = members.find((m) => m.id === 'me') ?? members[0] ?? null;
   const [isEditing, setIsEditing] = useState(false);
@@ -49,6 +51,22 @@ export function SettingsTab() {
     void i18n.changeLanguage(lang);
   };
 
+  const handleCurrencyChange = (code: CurrencyCode) => {
+    if (code === currency) return;
+    const tripExpenses = expenses.filter((e) => e.tripId === currentTripId);
+    if (
+      !confirmMixedCurrencyIfNeeded(
+        tripExpenses,
+        currency,
+        code,
+        t('history.mixed_currency_confirm'),
+      )
+    ) {
+      return;
+    }
+    setCurrency(code, true);
+  };
+
   return (
     <div className="animate-in fade-in slide-in-from-bottom-4 duration-500 space-y-8 pb-28">
       <section className="flex flex-col items-center text-center space-y-6">
@@ -215,7 +233,7 @@ export function SettingsTab() {
                   return (
                     <button
                       key={code}
-                      onClick={() => setCurrency(code, true)}
+                      onClick={() => handleCurrencyChange(code)}
                       className={cn(
                         'flex items-center gap-1.5 px-3 py-1 rounded-full text-xs font-medium transition-all cursor-pointer',
                         isActive
diff --git a/apps/split-meow/src/components/__tests__/Calculator.test.tsx b/apps/split-meow/src/components/__tests__/Calculator.test.tsx
index 7672cd962..b4174f6b2 100644
--- a/apps/split-meow/src/components/__tests__/Calculator.test.tsx
+++ b/apps/split-meow/src/components/__tests__/Calculator.test.tsx
@@ -1,5 +1,5 @@
 import { render, screen, fireEvent } from '@testing-library/react';
-import { describe, it, expect, beforeEach } from 'vitest';
+import { describe, it, expect, beforeEach, vi } from 'vitest';
 import { I18nextProvider } from 'react-i18next';
 import i18n from '../../i18n';
 import { Calculator } from '../Calculator';
@@ -102,6 +102,37 @@ describe('Calculator', () => {
     expect(useStore.getState().calculatorValue).toBe('');
   });
 
+  it('混幣記帳前需確認，取消則不儲存', () => {
+    const confirmSpy = vi.spyOn(window, 'confirm').mockReturnValue(false);
+    useStore.setState({
+      calculatorValue: '100',
+      currency: 'KRW',
+      currentTripId: 'default-trip',
+      expenses: [
+        {
+          id: 'exp-1',
+          tripId: 'default-trip',
+          type: 'split_evenly',
+          participantIds: ['me'],
+          paidBy: 'me',
+          totalAmount: 100,
+          perPersonAmounts: { me: 100 },
+          note: '',
+          createdAt: 1,
+          currency: 'TWD',
+        },
+      ],
+    });
+    renderCalc();
+    const allBtns = document.querySelectorAll('button');
+    const saveBtn = allBtns[allBtns.length - 1]!;
+    fireEvent.click(saveBtn);
+    expect(confirmSpy).toHaveBeenCalledWith(i18n.t('history.mixed_currency_confirm'));
+    expect(useStore.getState().expenses).toHaveLength(1);
+    expect(useStore.getState().calculatorValue).toBe('100');
+    confirmSpy.mockRestore();
+  });
+
   it('itemized 模式下無 focusedMemberId 時按鍵不更新值', () => {
     useStore.setState({
       splitMode: 'itemized',
diff --git a/apps/split-meow/src/components/__tests__/SettingsTab.test.tsx b/apps/split-meow/src/components/__tests__/SettingsTab.test.tsx
index 578942f82..646a875e3 100644
--- a/apps/split-meow/src/components/__tests__/SettingsTab.test.tsx
+++ b/apps/split-meow/src/components/__tests__/SettingsTab.test.tsx
@@ -1,5 +1,5 @@
 import { render, screen, fireEvent } from '@testing-library/react';
-import { describe, it, expect, beforeEach } from 'vitest';
+import { describe, it, expect, beforeEach, vi } from 'vitest';
 import { I18nextProvider } from 'react-i18next';
 import i18n from '../../i18n';
 import { SettingsTab } from '../SettingsTab';
@@ -85,4 +85,58 @@ describe('SettingsTab', () => {
       expect(useStore.getState().members).toHaveLength(3);
     }
   });
+
+  it('切換幣別若會混幣需確認，取消則不變更', () => {
+    const confirmSpy = vi.spyOn(window, 'confirm').mockReturnValue(false);
+    useStore.setState({
+      currency: 'TWD',
+      currentTripId: 'default-trip',
+      expenses: [
+        {
+          id: 'exp-1',
+          tripId: 'default-trip',
+          type: 'split_evenly',
+          participantIds: ['me'],
+          paidBy: 'me',
+          totalAmount: 100,
+          perPersonAmounts: { me: 100 },
+          note: '',
+          createdAt: 1,
+          currency: 'TWD',
+        },
+      ],
+    });
+    renderSettings();
+    fireEvent.click(screen.getByText('₩'));
+    expect(confirmSpy).toHaveBeenCalledWith(i18n.t('history.mixed_currency_confirm'));
+    expect(useStore.getState().currency).toBe('TWD');
+    confirmSpy.mockRestore();
+  });
+
+  it('切換幣別若會混幣且確認則變更', () => {
+    const confirmSpy = vi.spyOn(window, 'confirm').mockReturnValue(true);
+    useStore.setState({
+      currency: 'TWD',
+      currentTripId: 'default-trip',
+      expenses: [
+        {
+          id: 'exp-1',
+          tripId: 'default-trip',
+          type: 'split_evenly',
+          participantIds: ['me'],
+          paidBy: 'me',
+          totalAmount: 100,
+          perPersonAmounts: { me: 100 },
+          note: '',
+          createdAt: 1,
+          currency: 'TWD',
+        },
+      ],
+    });
+    renderSettings();
+    fireEvent.click(screen.getByText('₩'));
+    expect(confirmSpy).toHaveBeenCalledWith(i18n.t('history.mixed_currency_confirm'));
+    expect(useStore.getState().currency).toBe('KRW');
+    confirmSpy.mockRestore();
+  });
 });
diff --git a/apps/split-meow/src/config/__tests__/currencies.test.ts b/apps/split-meow/src/config/__tests__/currencies.test.ts
index 212e593dd..4083543f5 100644
--- a/apps/split-meow/src/config/__tests__/currencies.test.ts
+++ b/apps/split-meow/src/config/__tests__/currencies.test.ts
@@ -7,6 +7,7 @@ import {
   resolveExpenseCurrency,
   resolveTripCurrency,
   isMixedCurrencyTrip,
+  wouldCreateMixedCurrencyTrip,
   computeMemberBalances,
 } from '../currencies';
 
@@ -115,4 +116,18 @@ describe('trip currency helpers', () => {
     expect(resolveExpenseCurrency({}, 'KRW')).toBe('KRW');
     expect(resolveExpenseCurrency({ currency: 'TWD' }, 'KRW')).toBe('TWD');
   });
+
+  it('wouldCreateMixedCurrencyTrip 僅在單幣行程將混幣時為 true', () => {
+    expect(wouldCreateMixedCurrencyTrip([], 'TWD', 'KRW')).toBe(false);
+    expect(wouldCreateMixedCurrencyTrip([{ currency: 'TWD' }], 'TWD', 'TWD')).toBe(false);
+    expect(wouldCreateMixedCurrencyTrip([{ currency: 'TWD' }], 'TWD', 'KRW')).toBe(true);
+    expect(
+      wouldCreateMixedCurrencyTrip([{ currency: 'TWD' }, { currency: 'KRW' }], 'TWD', 'TWD'),
+    ).toBe(false);
+  });
+
+  it('wouldCreateMixedCurrencyTrip 舊資料缺 currency 欄位時仍偵測混幣風險', () => {
+    expect(wouldCreateMixedCurrencyTrip([{}], 'TWD', 'KRW')).toBe(true);
+    expect(wouldCreateMixedCurrencyTrip([{ currency: undefined }], 'TWD', 'KRW')).toBe(true);
+  });
 });
diff --git a/apps/split-meow/src/config/currencies.ts b/apps/split-meow/src/config/currencies.ts
index ffc271bca..0b25c079a 100644
--- a/apps/split-meow/src/config/currencies.ts
+++ b/apps/split-meow/src/config/currencies.ts
@@ -63,6 +63,34 @@ export function isMixedCurrencyTrip(
   return codes.size > 1;
 }
 
+/** 新增一筆 `incomingCurrency` 是否會使原本單幣行程變成混幣。 */
+export function wouldCreateMixedCurrencyTrip(
+  tripExpenses: { currency?: CurrencyCode }[],
+  globalCurrency: CurrencyCode,
+  incomingCurrency: CurrencyCode,
+): boolean {
+  if (tripExpenses.length === 0) return false;
+  const tripCurrency = resolveTripCurrency(tripExpenses, globalCurrency);
+  if (isMixedCurrencyTrip(tripExpenses, tripCurrency)) return false;
+  const lastExpense = tripExpenses[tripExpenses.length - 1];
+  if (lastExpense === undefined) return false;
+  const establishedCurrency = resolveExpenseCurrency(lastExpense, tripCurrency);
+  return incomingCurrency !== establishedCurrency;
+}
+
+/** 混幣風險時以 confirm 詢問；無風險或使用者確認則回傳 true。 */
+export function confirmMixedCurrencyIfNeeded(
+  tripExpenses: { currency?: CurrencyCode }[],
+  globalCurrency: CurrencyCode,
+  incomingCurrency: CurrencyCode,
+  confirmMessage: string,
+): boolean {
+  if (!wouldCreateMixedCurrencyTrip(tripExpenses, globalCurrency, incomingCurrency)) {
+    return true;
+  }
+  return window.confirm(confirmMessage);
+}
+
 /** 同幣別行程的成員餘額；僅在 `isMixedCurrencyTrip` 為 false 時有意義。 */
 export function computeMemberBalances(
   expenses: {
diff --git a/apps/split-meow/src/hooks/__tests__/useCurrencyAutoDetect.test.ts b/apps/split-meow/src/hooks/__tests__/useCurrencyAutoDetect.test.ts
new file mode 100644
index 000000000..20921f599
--- /dev/null
+++ b/apps/split-meow/src/hooks/__tests__/useCurrencyAutoDetect.test.ts
@@ -0,0 +1,68 @@
+import { renderHook, waitFor } from '@testing-library/react';
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import type * as CurrenciesModule from '../../config/currencies';
+import { useStore } from '../../store/useStore';
+
+vi.mock('../../lib/exchangeRate', () => ({
+  fetchMoneyboxRate: vi.fn().mockResolvedValue({ krwPerTwd: 43.5, updatedAt: '2026-01-01' }),
+}));
+
+vi.mock('../../config/currencies', async (importOriginal) => {
+  const actual = await importOriginal<typeof CurrenciesModule>();
+  return {
+    ...actual,
+    detectCurrencyFromTimezone: vi.fn(() => 'KRW' as const),
+  };
+});
+
+import { useCurrencyAutoDetect } from '../useCurrencyAutoDetect';
+import { detectCurrencyFromTimezone } from '../../config/currencies';
+
+describe('useCurrencyAutoDetect', () => {
+  beforeEach(() => {
+    vi.mocked(detectCurrencyFromTimezone).mockReturnValue('KRW');
+    useStore.setState({
+      currency: 'TWD',
+      currencyManuallySet: false,
+      currentTripId: 'default-trip',
+      expenses: [],
+    });
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('混幣風險時跳過自動切換幣別', async () => {
+    useStore.setState({
+      expenses: [
+        {
+          id: 'exp-1',
+          tripId: 'default-trip',
+          type: 'split_evenly',
+          participantIds: ['me'],
+          paidBy: 'me',
+          totalAmount: 100,
+          perPersonAmounts: { me: 100 },
+          note: '',
+          createdAt: 1,
+          currency: 'TWD',
+        },
+      ],
+    });
+
+    renderHook(() => useCurrencyAutoDetect());
+
+    await waitFor(() => {
+      expect(useStore.getState().currency).toBe('TWD');
+    });
+  });
+
+  it('無混幣風險時依時區自動切換', async () => {
+    renderHook(() => useCurrencyAutoDetect());
+
+    await waitFor(() => {
+      expect(useStore.getState().currency).toBe('KRW');
+    });
+  });
+});
diff --git a/apps/split-meow/src/hooks/useCurrencyAutoDetect.ts b/apps/split-meow/src/hooks/useCurrencyAutoDetect.ts
index c5e96c643..238fd165f 100644
--- a/apps/split-meow/src/hooks/useCurrencyAutoDetect.ts
+++ b/apps/split-meow/src/hooks/useCurrencyAutoDetect.ts
@@ -1,6 +1,6 @@
 import { useEffect } from 'react';
 import { useStore } from '../store/useStore';
-import { detectCurrencyFromTimezone } from '../config/currencies';
+import { detectCurrencyFromTimezone, wouldCreateMixedCurrencyTrip } from '../config/currencies';
 import { fetchMoneyboxRate } from '../lib/exchangeRate';
 
 /**
@@ -13,7 +13,8 @@ import { fetchMoneyboxRate } from '../lib/exchangeRate';
  */
 export function useCurrencyAutoDetect() {
   useEffect(() => {
-    const { currencyManuallySet, setCurrency, setExchangeRate } = useStore.getState();
+    const { currencyManuallySet, setCurrency, setExchangeRate, currency, expenses, currentTripId } =
+      useStore.getState();
 
     // 無論如何都更新匯率（供換算提示使用）
     fetchMoneyboxRate()
@@ -28,6 +29,11 @@ export function useCurrencyAutoDetect() {
     if (currencyManuallySet) return;
 
     const detected = detectCurrencyFromTimezone();
-    if (detected) setCurrency(detected, false);
+    if (!detected) return;
+
+    const tripExpenses = expenses.filter((e) => e.tripId === currentTripId);
+    if (wouldCreateMixedCurrencyTrip(tripExpenses, currency, detected)) return;
+
+    setCurrency(detected, false);
   }, []); // 僅 mount 時執行一次
 }
diff --git a/apps/split-meow/src/i18n.ts b/apps/split-meow/src/i18n.ts
index 2b7110a78..d3616838c 100644
--- a/apps/split-meow/src/i18n.ts
+++ b/apps/split-meow/src/i18n.ts
@@ -33,6 +33,8 @@ const resources = {
         members_count: '由 {{count}} 位探險家分攤',
         settlements: '結清方式',
         mixed_currency_warning: '此行程含多種幣別，無法合併總額與結算，請改用單一幣別記帳',
+        mixed_currency_confirm:
+          '此操作會使此行程混用多種幣別，之後無法合併總額與結算。確定要繼續嗎？',
         pay_to: '{{from}} 付給 {{to}}',
         balances: '各人結算',
         owed: '應收',
@@ -125,6 +127,8 @@ const resources = {
         settlements: 'How to Settle',
         mixed_currency_warning:
           'This trip mixes currencies; totals and settlement cannot be combined. Use a single currency.',
+        mixed_currency_confirm:
+          'This will mix currencies in this trip; totals and settlement cannot be combined afterward. Continue?',
         pay_to: '{{from}} pays {{to}}',
         balances: 'Balances',
         owed: 'Receives',
@@ -217,6 +221,8 @@ const resources = {
         settlements: '정산 방법',
         mixed_currency_warning:
           '이 여행에는 여러 통화가 섞여 있어 합계와 정산을 합칠 수 없습니다. 단일 통화를 사용하세요.',
+        mixed_currency_confirm:
+          '이 작업으로 이 여행에 여러 통화가 섞여, 이후 합계와 정산을 합칠 수 없습니다. 계속하시겠습니까?',
         pay_to: '{{from}} → {{to}}',
         balances: '개인 정산',
         owed: '받을 돈',
@@ -309,6 +315,8 @@ const resources = {
         settlements: '精算方法',
         mixed_currency_warning:
           'この旅行は複数の通貨が混在しており、合計と精算を統合できません。単一通貨を使用してください。',
+        mixed_currency_confirm:
+          'この操作により、この旅行で複数の通貨が混在し、以後合計と精算を統合できなくなります。続行しますか？',
         pay_to: '{{from}} → {{to}}',
         balances: '各自の精算',
         owed: '受け取り',