diff --git a/AGENTS.md b/AGENTS.md
index 3f6dc41..3622f5f 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -2179,8 +2179,9 @@ similar are internal endpoints/hook names, not credits — leave them.)
   pages (`common`, `nav`, `chromeUi`, `desktopNav`, `navSheet`, `pwa`, `home`,
   `about`, `faq`, `onboarding`); more namespaces land in later batches.
 - **Partial by design:** a missing `th` key falls back to English. `check:i18n`
-  is now **3-way** — en/es stay key-for-key identical (unchanged), `th` may be
-  partial and is reported as **coverage %**; a `th` key absent from `en` fails.
+  is now **multi-way** — en/es stay key-for-key identical (unchanged), `th` (and
+  `ja`) may be partial and are reported as **coverage %**; a `th`/`ja` key absent
+  from `en` fails.
 - **Font:** `@fontsource/kanit` (Kanit) imported in `main.tsx` and added to the
   CSS + Tailwind font stacks — ⚠️ Outfit/Inter/Bebas/Barlow have **no Thai
   glyphs**, so without it Thai silently falls back to a system font.
@@ -2189,6 +2190,23 @@ similar are internal endpoints/hook names, not credits — leave them.)
 - **Do-not-translate is broader than tricks for Thai:** core skate vocabulary
   stays English (spot, park, bowl, DIY, vert, crew, line, Stash, The Lab, Nostr,
   Lightning, Zap, Relay, sats) because Thai skaters use the English terms.
+
+### Japanese (日本語) — fourth locale (2026-10-07)
+- Added `ja` alongside en/es/th on the **same framework**. Translations live in
+  **`src/i18n/ja/`** as fragments composed in `index.ts`. Batch 1 = `common`,
+  `nav`, `pwa`, `home`; more namespaces land in later batches.
+- **Partial by design** (same as `th`): a missing `ja` key falls back to English.
+  `check-i18n.mjs` reports `ja` coverage and fails if a `ja` key is absent from `en`.
+- **Font:** `@fontsource/noto-sans-jp` (Noto Sans JP) imported in `main.tsx` and
+  added to the CSS + Tailwind font stacks — ⚠️ Outfit/Inter/Bebas/Barlow/Kanit have
+  **no Japanese glyphs**, so without it Japanese falls back to a system font.
+- **Detection:** a Japanese-device browser (`ja*`) starts in `ja` (`detectLanguage`).
+- **Katakana/English for skate vocabulary** (spot→スポット, park→パーク, crew→クルー;
+  Stash/The Lab/Nostr/Lightning/Zap/sats stay English). Trick names stay English.
+- **Castellano term change (2026-10-07):** crew is **"peña"** in `es.ts`, not
+  "cuadrilla" — a case-preserving replace of 158 occurrences (`Cuadrilla→Peña`,
+  `cuadrilla→peña`, `CUADRILLA→PEÑA`). ⚠️ Do **not** use `perl -CSD` on these files
+  (it mojibakes UTF-8); use a UTF-8-safe edit/Node.
 - The app ships **English + Spanish (Castellano, Spain)**. The framework already
   exists: `src/contexts/language.ts` (context/types/hooks) + `LanguageContext.tsx`
   (provider), `src/i18n/en.ts` + `src/i18n/es.ts` (one default-export object
diff --git a/package-lock.json b/package-lock.json
index 71384de..f9cc924 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -21,6 +21,7 @@
         "@fontsource/barlow-condensed": "^5.2.8",
         "@fontsource/bebas-neue": "^5.2.7",
         "@fontsource/kanit": "^5.3.0",
+        "@fontsource/noto-sans-jp": "^5.3.0",
         "@getalby/sdk": "^5.1.1",
         "@hookform/resolvers": "^3.9.0",
         "@modelcontextprotocol/sdk": "^1.29.0",
@@ -1511,6 +1512,15 @@
         "url": "https://github.com/sponsors/ayuhito"
       }
     },
+    "node_modules/@fontsource/noto-sans-jp": {
+      "version": "5.3.0",
+      "resolved": "https://registry.npmjs.org/@fontsource/noto-sans-jp/-/noto-sans-jp-5.3.0.tgz",
+      "integrity": "sha512-OZbBzZ8LrFRs2RFT2Cc0HUV2C2ZeSfyEXhDJWR0EZuZaraJfRubpvrFmUnCljuBDEkw8Vn6CE/+nf9dj9b+0lg==",
+      "license": "OFL-1.1",
+      "funding": {
+        "url": "https://github.com/sponsors/ayuhito"
+      }
+    },
     "node_modules/@getalby/lightning-tools": {
       "version": "5.2.0",
       "resolved": "https://registry.npmjs.org/@getalby/lightning-tools/-/lightning-tools-5.2.0.tgz",
diff --git a/package.json b/package.json
index 94d7375..d6ae91e 100644
--- a/package.json
+++ b/package.json
@@ -41,6 +41,7 @@
     "@fontsource/barlow-condensed": "^5.2.8",
     "@fontsource/bebas-neue": "^5.2.7",
     "@fontsource/kanit": "^5.3.0",
+    "@fontsource/noto-sans-jp": "^5.3.0",
     "@getalby/sdk": "^5.1.1",
     "@hookform/resolvers": "^3.9.0",
     "@modelcontextprotocol/sdk": "^1.29.0",
diff --git a/scripts/check-i18n.mjs b/scripts/check-i18n.mjs
index 457a59d..cbd1839 100644
--- a/scripts/check-i18n.mjs
+++ b/scripts/check-i18n.mjs
@@ -102,6 +102,7 @@ async function main() {
   const en = flatten(await loadTranslations(join(SRC, 'i18n/en.ts')));
   const es = flatten(await loadTranslations(join(SRC, 'i18n/es.ts')));
   const th = flatten(await loadTranslations(join(SRC, 'i18n/th/index.ts')));
+  const ja = flatten(await loadTranslations(join(SRC, 'i18n/ja/index.ts')));
 
   const problems = [];
 
@@ -123,6 +124,11 @@ async function main() {
   for (const key of th) if (!en.has(key)) problems.push(`th has a key not in en: ${key}`);
   const thCoverage = en.size ? Math.round(((en.size - missingTh) / en.size) * 100) : 100;
 
+  // 4. Japanese may also be PARTIAL (same rules as Thai).
+  const missingJa = [...en].filter((k) => !ja.has(k)).length;
+  for (const key of ja) if (!en.has(key)) problems.push(`ja has a key not in en: ${key}`);
+  const jaCoverage = en.size ? Math.round(((en.size - missingJa) / en.size) * 100) : 100;
+
   if (problems.length) {
     console.error(`\n✖ i18n check failed (${problems.length}):`);
     for (const p of problems.slice(0, 200)) console.error(`  • ${p}`);
@@ -130,7 +136,7 @@ async function main() {
     process.exit(1);
   }
 
-  console.log(`✓ i18n check passed — ${en.size} keys in en/es, ${used.size} used · th coverage ${thCoverage}% (${en.size - missingTh}/${en.size})`);
+  console.log(`✓ i18n check passed — ${en.size} keys in en/es, ${used.size} used · th ${thCoverage}% (${en.size - missingTh}/${en.size}) · ja ${jaCoverage}% (${en.size - missingJa}/${en.size})`);
 }
 
 main().catch((err) => {
diff --git a/src/components/LanguageToggle.tsx b/src/components/LanguageToggle.tsx
index d9604b9..16ebd37 100644
--- a/src/components/LanguageToggle.tsx
+++ b/src/components/LanguageToggle.tsx
@@ -20,6 +20,7 @@ const languages: LanguageOption[] = [
   { code: 'en', name: 'English', nativeName: 'English', flag: '🇬🇧' },
   { code: 'es', name: 'Spanish', nativeName: 'Español', flag: '🇪🇸' },
   { code: 'th', name: 'Thai', nativeName: 'ไทย', flag: '🇹🇭' },
+  { code: 'ja', name: 'Japanese', nativeName: '日本語', flag: '🇯🇵' },
 ];
 
 interface LanguageToggleProps {
diff --git a/src/components/SasquatchGuide.tsx b/src/components/SasquatchGuide.tsx
index 7dc5354..df4d572 100644
--- a/src/components/SasquatchGuide.tsx
+++ b/src/components/SasquatchGuide.tsx
@@ -1115,7 +1115,9 @@ export function SasquatchGuide({ message, onDismiss }: SasquatchGuideProps) {
           ? '\n\nAlways reply in Spanish (Castellano). Keep skateboarding trick names in English (kickflip, ollie, nollie, heelflip, etc.) and never translate them.'
           : language === 'th'
             ? '\n\nAlways reply in Thai (ไทย) with a friendly, casual skate tone. Keep skateboarding trick names in English (kickflip, ollie, nollie, heelflip, etc.) and core skate vocabulary (spot, park, bowl, DIY, vert, crew, line) in English — never translate those.'
-            : '';
+            : language === 'ja'
+              ? '\n\nAlways reply in Japanese (日本語) with a friendly, casual skate tone. Keep skateboarding trick names in English (kickflip, ollie, nollie, heelflip, etc.) and core skate vocabulary in English or katakana (spot, park, bowl, DIY, vert, crew, line) — never translate the trick names.'
+              : '';
         const systemPrompt = (personal.available && isPro
           ? `${SYSTEM_PROMPT}${PERSONAL_TOOLS_PROMPT}`
           : SYSTEM_PROMPT) + languageDirective;
diff --git a/src/contexts/LanguageContext.tsx b/src/contexts/LanguageContext.tsx
index 4123146..ea62d2e 100644
--- a/src/contexts/LanguageContext.tsx
+++ b/src/contexts/LanguageContext.tsx
@@ -23,6 +23,7 @@ function detectLanguage(): Language {
   if (typeof navigator === 'undefined') return 'en';
   const tags = [navigator.language, ...(navigator.languages ?? [])].map((t) => t?.toLowerCase() ?? '');
   if (tags.some((t) => t.startsWith('th'))) return 'th';
+  if (tags.some((t) => t.startsWith('ja'))) return 'ja';
   if (tags.some((t) => t.startsWith('es'))) return 'es';
   return 'en';
 }
diff --git a/src/contexts/language.ts b/src/contexts/language.ts
index 59186a0..79ec37f 100644
--- a/src/contexts/language.ts
+++ b/src/contexts/language.ts
@@ -2,6 +2,7 @@ import { createContext, useContext } from 'react';
 import en from '@/i18n/en';
 import es from '@/i18n/es';
 import th from '@/i18n/th';
+import ja from '@/i18n/ja';
 
 /**
  * Language context + types + hooks. Split from the `LanguageProvider` component
@@ -9,7 +10,7 @@ import th from '@/i18n/th';
  * `react-refresh/only-export-components` rule.
  */
 
-export type Language = 'en' | 'es' | 'th';
+export type Language = 'en' | 'es' | 'th' | 'ja';
 
 type TranslationKey = string;
 type TranslationValue = string | { [key: string]: TranslationValue };
@@ -31,7 +32,7 @@ export interface LanguageContextType {
 
 export const LanguageContext = createContext<LanguageContextType | undefined>(undefined);
 
-export const translations: Record<Language, Translations> = { en, es, th };
+export const translations: Record<Language, Translations> = { en, es, th, ja };
 
 /** Get a nested value from an object using dot notation (`nav.home`). */
 export function getNestedValue(obj: Translations, path: string): string | undefined {
diff --git a/src/i18n/ja/batch1.ts b/src/i18n/ja/batch1.ts
new file mode 100644
index 0000000..8a61f95
--- /dev/null
+++ b/src/i18n/ja/batch1.ts
@@ -0,0 +1,150 @@
+/**
+ * Japanese (日本語) — batch 1: app shell + static pages.
+ *
+ * Partial by design: any key not yet translated falls back to English, and
+ * `scripts/check-i18n.mjs` reports coverage. Skate/brand vocabulary stays
+ * English or katakana (Spot→スポット, Park→パーク, Crew→クルー, Stash, The Lab,
+ * Nostr, Lightning, Zap, sats), because Japanese skaters use those.
+ */
+const jaBatch1 = {
+  common: {
+    loading: '読み込み中...',
+    error: 'エラー',
+    save: '保存',
+    cancel: 'キャンセル',
+    delete: '削除',
+    edit: '編集',
+    add: '追加',
+    remove: '削除',
+    create: '作成',
+    submit: '送信',
+    close: '閉じる',
+    back: '戻る',
+    next: '次へ',
+    done: '完了',
+    search: '検索',
+    filter: '絞り込み',
+    sort: '並べ替え',
+    share: 'シェア',
+    more: 'もっと見る',
+    copy: 'コピー',
+    copied: 'コピーしました!',
+    viewAll: 'すべて表示',
+    seeMore: 'もっと見る',
+    showMore: 'もっと表示',
+    showLess: '表示を減らす',
+    noResults: '結果が見つかりません',
+    tryAgain: 'もう一度試す',
+    learnMore: '詳しく見る',
+    getStarted: 'はじめる',
+    yes: 'はい',
+    no: 'いいえ',
+    or: 'または',
+    and: 'と',
+    free: '無料',
+    new: '新着',
+    popular: '人気',
+    featured: '注目',
+    verified: '認証済み',
+    sats: 'sats',
+    clear: 'クリア',
+    clearFilters: 'フィルターをクリア',
+    clearAll: 'すべてクリア',
+    hide: '非表示',
+    undo: '元に戻す',
+  },
+  nav: {
+    home: 'ホーム',
+    map: 'スポット',
+    lab: 'The Lab',
+    events: 'イベント',
+    crews: 'クルー',
+    wallet: 'ウォレット',
+    settings: '設定',
+    more: 'もっと見る',
+    rewards: 'リワード',
+    stats: 'コミュニティ統計',
+    about: '概要',
+    food: 'フード',
+    marketplace: 'マーケットプレイス',
+    mints: 'ミント',
+    appShop: 'アプリショップ',
+    diy: 'DIY',
+    trades: 'マーケットプレイス',
+    stash: 'Stash',
+    collections: 'コレクション',
+    contacts: '連絡先',
+    messages: 'メッセージ',
+    skaters: 'スケーター',
+    glossary: '用語集',
+    riderCard: 'ライダーカード',
+    supporters: 'サポーター ⚡',
+    newsletter: 'ニュースレター',
+    viewProfile: 'プロフィールを見る',
+    leaderboards: 'リーダーボード',
+  },
+  pwa: {
+    installTitle: 'SkateSpotsをインストール',
+    installSubtitle: 'アプリの全機能を体験',
+    addToHomeScreen: 'ホーム画面に追加',
+    installApp: 'アプリをインストール',
+    maybeLater: 'あとで',
+    gotIt: '了解!',
+    benefits: {
+      fast: '超高速',
+      offline: 'オフライン対応',
+      fullAccess: '全機能',
+    },
+    iosStep1: '共有ボタンをタップ',
+    iosStep1Hint: 'Safariの下部にあります',
+    iosStep2: 'スクロールして「ホーム画面に追加」をタップ',
+    iosStep2Hint: '共有メニュー内',
+    iosStep3: '「追加」をタップしてインストール',
+    iosStep3Hint: 'ホーム画面にアプリが表示されます',
+    androidStep1: 'メニューボタンをタップ',
+    androidStep1Hint: 'ブラウザの(三点)メニュー',
+    androidStep2: '「ホーム画面に追加」を選択',
+    androidStep2Hint: 'または「アプリをインストール」',
+    androidStep3: '確認してインストール',
+    androidStep3Hint: 'ホーム画面にアプリが表示されます',
+    tagline: '無料 • オフライン対応 • 高速で信頼できる',
+  },
+  home: {
+    tagline: 'コミュニティ主導 • リレー基盤',
+    title: 'SKATE SPOTS',
+    subtitle: 'スポットを発見 • 集まる • トリックを学ぶ • 地元を応援',
+    description: '本当のスケートはオフラインで起きる — このツールはそれを見つける手助けをします。',
+    betaWarning: 'ベータ - 開発中 • 自己責任で利用してください',
+    pwaHint: 'プログレッシブウェブアプリ - 端末にインストール',
+    exploreMap: 'マップを見る',
+    addSpot: 'スポットを追加',
+    globalSpots: '世界中のスポット',
+    openCommunity: 'オープンなコミュニティ',
+    lightningZaps: 'Lightning Zaps',
+    featuredSpots: '注目のスポット',
+    featuredSpotsDesc: '世界中のコミュニティが認証したスケートスポット',
+    upcomingEvents: '開催予定のイベント',
+    upcomingEventsDesc: '近日開催のコンテスト、ミートアップ、セッション',
+    noSpotsYet: 'まだスポットがありません',
+    noSpotsHint: '最初のスケートスポットを追加しましょう!',
+    noEventsYet: '予定されたイベントはありません',
+    noEventsHint: 'また後で確認するか、作成しましょう!',
+    theLab: 'The Lab',
+    theLabDesc: 'トリックを学び、コーチを見つけて、スケートを上達させよう',
+    coaches: 'コーチ',
+    coachesDesc: '上達を手助けしてくれる経験豊富なスケーターから学ぶ',
+    filmers: 'フィルマー',
+    filmersDesc: 'フィルマーとつながって最高のクリップを撮ろう',
+    findCoach: 'コーチを探す',
+    becomeCoach: 'コーチになる',
+    findFilmer: 'フィルマーを探す',
+    becomeFilmer: 'フィルマーになる',
+    viewAllSpots: 'すべてのスポットを見る',
+    viewAllEvents: 'すべてのイベントを見る',
+    createEvent: 'イベントを作成',
+    readyToShare: 'シェアする準備はできた?',
+    readyToShareDesc: 'お気に入りのスポットを追加してスケートコミュニティを育てましょう。あなたがシェアする一つ一つが、仲間が新しい場所を見つける助けになります。',
+  },
+};
+
+export default jaBatch1;
diff --git a/src/i18n/ja/index.ts b/src/i18n/ja/index.ts
new file mode 100644
index 0000000..89dc00e
--- /dev/null
+++ b/src/i18n/ja/index.ts
@@ -0,0 +1,15 @@
+import batch1 from './batch1';
+
+/**
+ * Japanese (日本語) translations — composed from fragments so parallel batches
+ * never touch one giant file (same pattern as `i18n/th/`).
+ *
+ * Partial by design: any key not yet translated falls back to English, and
+ * `scripts/check-i18n.mjs` reports the coverage. Add new fragments here as the
+ * remaining namespaces land.
+ */
+const ja = {
+  ...batch1,
+};
+
+export default ja;
diff --git a/src/index.css b/src/index.css
index 2ce8f1c..e50364a 100644
--- a/src/index.css
+++ b/src/index.css
@@ -149,18 +149,18 @@
 /* ── Display font utilities ───────────────────────────────────────────────── */
 /* Bebas Neue — used for all major headlines */
 .font-display {
-  font-family: 'Bebas Neue', 'Kanit', 'Impact', 'Arial Black', sans-serif;
+  font-family: 'Bebas Neue', 'Kanit', 'Noto Sans JP', 'Impact', 'Arial Black', sans-serif;
   letter-spacing: 0.04em;
 }
 
 /* Barlow Condensed — used for subheadings, labels, nav */
 .font-condensed {
-  font-family: 'Barlow Condensed', 'Kanit', 'Arial Narrow', sans-serif;
+  font-family: 'Barlow Condensed', 'Kanit', 'Noto Sans JP', 'Arial Narrow', sans-serif;
 }
 
 .neon-header {
   animation: neon-glow 2s ease-in-out infinite;
-  font-family: 'Bebas Neue', 'Kanit', 'Impact', 'Arial Black', sans-serif;
+  font-family: 'Bebas Neue', 'Kanit', 'Noto Sans JP', 'Impact', 'Arial Black', sans-serif;
   letter-spacing: 0.06em;
 }
 
@@ -714,7 +714,7 @@ select {
   position: relative;
   overflow: hidden;
   flex-shrink: 0;
-  font-family: 'Inter Variable', 'Inter', 'Kanit', system-ui, sans-serif;
+  font-family: 'Inter Variable', 'Inter', 'Kanit', 'Noto Sans JP', system-ui, sans-serif;
   box-shadow: 0 8px 40px rgba(0,0,0,0.6), 0 2px 8px rgba(0,0,0,0.4);
 }
 
diff --git a/src/lib/dateFnsI18n.ts b/src/lib/dateFnsI18n.ts
index 4590eb1..9962e5e 100644
--- a/src/lib/dateFnsI18n.ts
+++ b/src/lib/dateFnsI18n.ts
@@ -19,7 +19,7 @@ import {
   formatDistance as _formatDistance,
   format as _format,
 } from 'date-fns';
-import { enUS, es as esLocale, th as thLocale } from 'date-fns/locale';
+import { enUS, es as esLocale, th as thLocale, ja as jaLocale } from 'date-fns/locale';
 import type { Locale } from 'date-fns';
 import type { Language } from '@/contexts/language';
 
@@ -34,6 +34,9 @@ export function setDateLocale(language: Language): void {
   } else if (language === 'th') {
     activeLocale = thLocale;
     activeCode = 'th-TH';
+  } else if (language === 'ja') {
+    activeLocale = jaLocale;
+    activeCode = 'ja-JP';
   } else {
     activeLocale = enUS;
     activeCode = 'en-US';
diff --git a/src/main.tsx b/src/main.tsx
index 1afdd2c..f74b121 100644
--- a/src/main.tsx
+++ b/src/main.tsx
@@ -39,6 +39,10 @@ import '@fontsource/kanit/400.css';
 import '@fontsource/kanit/500.css';
 import '@fontsource/kanit/600.css';
 import '@fontsource/kanit/700.css';
+// Japanese glyphs — Noto Sans JP (Outfit/Inter/Bebas/Barlow/Kanit have none)
+import '@fontsource/noto-sans-jp/400.css';
+import '@fontsource/noto-sans-jp/500.css';
+import '@fontsource/noto-sans-jp/700.css';
 
 // Leaflet CSS is loaded via CDN in index.html
 
diff --git a/tailwind.config.ts b/tailwind.config.ts
index cb44f2b..51405f5 100644
--- a/tailwind.config.ts
+++ b/tailwind.config.ts
@@ -28,7 +28,7 @@ export default {
 		},
 		extend: {
 			fontFamily: {
-				sans: ['Outfit Variable', 'Outfit', 'Kanit', 'system-ui', 'sans-serif'],
+				sans: ['Outfit Variable', 'Outfit', 'Kanit', 'Noto Sans JP', 'system-ui', 'sans-serif'],
 			},
 			colors: {
 				border: 'hsl(var(--border))',