diff --git a/docs/CURRENT-STATE.md b/docs/CURRENT-STATE.md
index c55031a6a..0a71ce3e2 100644
--- a/docs/CURRENT-STATE.md
+++ b/docs/CURRENT-STATE.md
@@ -3733,7 +3733,7 @@ test:track` 43 tracking invariants · headless QA harnesses under
 
 ### Interface localization (October 2026)
 
-Shipped locales: **en** (source and fallback), **es**, **ru**, **zh-CN** and **zh-Hant**. The code lives in
+Shipped locales: **en** (source and fallback), **es**, **ja**, **ru**, **zh-CN** and **zh-Hant**. The code lives in
 `src/i18n/`. How to mark UI text: [`docs/I18N.md`](./I18N.md); translator workflow:
 [`docs/TRANSLATORS.md`](./TRANSLATORS.md).
 
diff --git a/docs/TRANSLATORS.md b/docs/TRANSLATORS.md
index 830ca155e..95d7a6088 100644
--- a/docs/TRANSLATORS.md
+++ b/docs/TRANSLATORS.md
@@ -1,6 +1,6 @@
 # Translating God's Eye View
 
-Shipped languages: **English** (source and fallback), **Spanish** (`es`), **Russian** (`ru`), **Simplified Chinese** (`zh-CN`) and **Traditional Chinese** (`zh-Hant`).
+Shipped languages: **English** (source and fallback), **Spanish** (`es`), **Japanese** (`ja`), **Russian** (`ru`), **Simplified Chinese** (`zh-CN`) and **Traditional Chinese** (`zh-Hant`).
 
 ## How it works
 
@@ -63,4 +63,4 @@ These are contracts, not copy: layer ids and registry keys, status enums and oth
 
 ## Language notes
 
-Each language keeps its glossary, plural forms, units and style in its own file, `docs/translations/<code>.md`: [Spanish](./translations/es.md), [Russian](./translations/ru.md), [Simplified Chinese](./translations/zh-CN.md), [Traditional Chinese](./translations/zh-Hant.md). Read it before translating, and add a term there when you settle on one.
+Each language keeps its glossary, plural forms, units and style in its own file, `docs/translations/<code>.md`: [Spanish](./translations/es.md), [Japanese](./translations/ja.md), [Russian](./translations/ru.md), [Simplified Chinese](./translations/zh-CN.md), [Traditional Chinese](./translations/zh-Hant.md). Read it before translating, and add a term there when you settle on one.
diff --git a/docs/translations/ja.md b/docs/translations/ja.md
new file mode 100644
index 000000000..74abc49b5
--- /dev/null
+++ b/docs/translations/ja.md
@@ -0,0 +1,90 @@
+# Japanese (`ja`)
+
+Catalog: `src/i18n/locales/ja.json`. General rules: [TRANSLATORS.md](../TRANSLATORS.md).
+
+Translation source: abiyeenzo (#913).
+
+## Style
+
+- Sentences use です／ます: 「キーを保存しました」, 「もう一度お試しください」. Labels, buttons and HUD chips are short nouns or verb stems without a final 。: 「再試行」, 「保存」, 「ライブ」, 「オフ」.
+- Japanese has no case: an UPPERCASE English label becomes the same short Japanese label. Codes, units and names stay in their own case (HDG, ADS-B, km).
+- Full-width punctuation in Japanese text: 、。：？！（）「」. Quote names and titles with 「」: 「{shot}」を削除しますか？ Keep `·` as the HUD separator, `…` for an ellipsis and `—`, `→`, `↗` as in English. A dash inside a sentence becomes 。 or ：.
+- Button names cited in running text go in ［］: ［次へ］を使ってください.
+- No space between Japanese and Latin letters, digits or a `{placeholder}`: 「AIS船舶」, 「Spaceを長押しで話す」, 「{count}件」, 「{name}を表示」.
+- Two exceptions keep the space: a Japanese label before its value in a readout (「雲量 {pct}%」, 「ステーション {id}」, 「高度 {altitude}」), and the state words in toggles (「CCTV オン」, 「ラジオ オフ」).
+- Loanwords are katakana with the long-vowel mark, as in Microsoft and Apple style: ユーザー, ブラウザー, ビューアー, プロバイダー, レイヤー, フィルター, モニター, パラメーター. Pick one katakana form per term and keep it (see the glossary).
+- Product, provider and dataset names stay in Latin script: Google Maps, Cesium ion, OpenSky, CelesTrak, NASA FIRMS, Sentinel-2, KartaView. Google's own product names use Google's Japanese: Google ストリートビュー. Nepal place names in the Bhote Koshi scenes stay in Latin script, as in zh-CN; country and well-known city names are translated (ネパール, 東京, オースティン).
+
+## Numbers, units and dates
+
+- Units: km, m, km/h, m/s, kt, ft, mph, hPa, MW, dB, °, % stay as written, with the English spacing (「250 km」, 「{frp} MW」, 「95 kt」).
+- Time units are words written straight after the number: 秒, 分, 時間, 日 (「5分前」, 「{count}時間」, 「1時間5分」). `formatQuantity` and `formatRelativeTime` drop the space for Japanese unit words, so code-built values match the catalog.
+- Counters follow the thing counted: 機 (aircraft), 隻 (ships), 基 (satellites), 台 (cameras, vehicles), 局 (stations), 件 (records, results, events), か所 (sites), 個 (generic). Prefer a counter to a bare number.
+- Dates come from `Intl` and read year first: 2026年8月26日. Coordinates in prose are 北緯 / 東経; HUD hemisphere letters stay N, S, E, W.
+
+## Plurals
+
+`Intl.PluralRules('ja')` has one category, `other`: a `tn()` entry has only that form. Put the counter next to the number: 「航空機{n}機」, 「船舶{n}隻」, 「メッセージ {count}件」.
+
+## Glossary
+
+| English                                   | 日本語                                  |
+| ----------------------------------------- | --------------------------------------- |
+| layer / data layer                        | レイヤー / データレイヤー               |
+| feed                                      | フィード                                |
+| source / data source / attribution        | ソース / データソース / 出典            |
+| LIVE (status) / live data                 | ライブ / ライブデータ                   |
+| SIMULATED                                 | シミュレーション                        |
+| stale / degraded / unavailable            | 更新遅延 / 性能低下 / 利用不可          |
+| retry                                     | 再試行                                  |
+| key (API key) / POWER UP                  | キー（APIキー）/ パワーアップ           |
+| Provider Settings                         | プロバイダー設定                        |
+| globe / map / map source / basemap        | 地球儀 / 地図 / マップソース / ベースマップ |
+| mapped (OSM-mapped sites)                 | マップ済み                              |
+| aircraft / flight / military flights      | 航空機 / フライト / 軍用機              |
+| vessel, ship                              | 船舶                                    |
+| satellite                                 | 衛星                                    |
+| launch (rocket) / space mission           | 打ち上げ / 宇宙ミッション               |
+| stage (rocket) / downrange                | 段 / ダウンレンジ                       |
+| contact (tracked object)                  | コンタクト                              |
+| installation, site                        | 施設                                    |
+| camera / CCTV camera                      | カメラ / CCTVカメラ                     |
+| earthquake / active fire / fire perimeter | 地震 / 火災検知 / 延焼範囲              |
+| contained (fire)                          | 鎮圧                                    |
+| scene / shot                              | シーン / ショット                       |
+| briefing / panel / dock                   | ブリーフィング / パネル / ドック        |
+| preset / visual preset                    | プリセット / ビジュアルプリセット       |
+| Tactical / Operator / Minimal / Cyber     | タクティカル / オペレーター / ミニマル / サイバー |
+| Scope (circular viewport mask) / keyhole  | スコープ / キーホール                   |
+| Clean UI                                  | クリーンUI                              |
+| cockpit                                   | コックピット                            |
+| heading / altitude / ground speed         | 機首方位 / 高度 / 対地速度              |
+| HDG / BRG / CRS / FOV                     | そのまま（HUDコード）                   |
+| squawk                                    | スコーク                                |
+| map heading / North up                    | 方位 / 北を上に                         |
+| Street Level / Street View (Google)       | ストリートレベル / ストリートビュー     |
+| Recent Imagery                            | 最近の衛星画像                          |
+| Global Context / Context (panel)          | グローバルコンテキスト / コンテキスト   |
+| radio (station) / tune / seek             | ラジオ（局）/ 選局・同調 / シーク       |
+| transit / bikeshare                       | 公共交通 / シェアサイクル               |
+| Previous / Next                           | 前へ / 次へ                             |
+| compass points (N, NNE, NE, ENE, …)       | 北, 北北東, 北東, 東北東, …             |
+| weather field / FIELD (selector)          | 要素                                    |
+| weather model / forecast lead hours       | モデル / 予報時間                       |
+| valid (forecast time) / issued            | 対象時刻 / 発表                         |
+| radar reflectivity / lightning density    | レーダー反射強度 / 雷密度               |
+| cyclone / advisory (NHC/CPHC)             | サイクロン / 情報（第N報）              |
+| forecast uncertainty cone                 | 予報円                                  |
+| tropical storm / tropical depression      | 熱帯暴風雨 / 熱帯低気圧                 |
+| scene cloud / mosaic / daily overview     | シーン雲量 / モザイク / 日次概観        |
+| box (Recent Imagery area)                 | 範囲                                    |
+| pin (Recent Imagery)                      | 固定                                    |
+| outline / mark (map annotation)           | 輪郭 / マーク                           |
+| share (a view or link)                    | 共有                                    |
+| AI agent                                  | AIエージェント                          |
+| frame (bring into view)                   | 視野に収める                            |
+| story beat / evidence beat                | ストーリー区切り / 証拠の区切り         |
+| camera pose / camera move                 | カメラの姿勢 / 移動                     |
+| roundabout / ramp / exit                  | ロータリー / ランプ / 出口              |
+| reduced motion                            | 動きを減らす                            |
+| NOT AN ALL-CLEAR                          | 安全の保証ではありません                |
diff --git a/scripts/package-boundaries.json b/scripts/package-boundaries.json
index 9f06f73f6..89c2596b5 100644
--- a/scripts/package-boundaries.json
+++ b/scripts/package-boundaries.json
@@ -440,6 +440,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -480,6 +481,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -510,6 +512,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -532,6 +535,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -549,6 +553,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -569,6 +574,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -596,6 +602,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -631,6 +638,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -648,6 +656,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -807,6 +816,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -929,6 +939,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -997,6 +1008,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -1046,6 +1058,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1109,6 +1122,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1139,6 +1153,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -1170,6 +1185,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1201,6 +1217,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -1235,6 +1252,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1278,6 +1296,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -1315,6 +1334,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -1366,6 +1386,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1396,6 +1417,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1445,6 +1467,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1509,6 +1532,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -1539,6 +1563,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1580,6 +1605,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1603,6 +1629,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1660,6 +1687,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -1723,6 +1751,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -2606,6 +2635,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -3191,6 +3221,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -3302,6 +3333,7 @@
       "src/data/cameraArrival.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -3326,6 +3358,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -3363,6 +3396,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -3390,6 +3424,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -3482,6 +3517,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -3538,6 +3574,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -3793,6 +3830,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -3837,6 +3875,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -3891,6 +3930,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json"
@@ -3908,6 +3948,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -3968,6 +4009,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
@@ -4165,6 +4207,7 @@
       "src/i18n/index.js",
       "src/i18n/locale.js",
       "src/i18n/locales/es.json",
+      "src/i18n/locales/ja.json",
       "src/i18n/locales/ru.json",
       "src/i18n/locales/zh-CN.json",
       "src/i18n/locales/zh-Hant.json",
diff --git a/src/data/directions.test.mjs b/src/data/directions.test.mjs
index bf6e2878e..38bada296 100644
--- a/src/data/directions.test.mjs
+++ b/src/data/directions.test.mjs
@@ -875,6 +875,13 @@ test('route numbers follow the page language and never break from their unit', (
   );
   const list = directionsStepList({ route });
   assert.equal(list.items[1].lead, '240\u00a0м');
+
+  // Japanese unit words follow the number: 1時間5分, but 1.5 km.
+  setLocale('ja');
+  assert.equal(routeDistanceText(1500), '1.5\u00a0km');
+  assert.equal(routeDurationText(39), '39秒');
+  assert.equal(routeDurationText(3900), '1時間5分');
+  assert.equal(routeDurationText(7200), '2時間');
 });
 
 test('ramp and lane wording stays localized after choosing the maneuver form', (t) => {
@@ -892,6 +899,12 @@ test('ramp and lane wording stays localized after choosing the maneuver form', (
       'Toma la salida bruscamente a la derecha',
       'Toma el carril izquierdo',
     ],
+    [
+      'ja',
+      'ランプを斜め左に進んでI-95に入る',
+      '出口を急角度で右に出る',
+      '左車線を通行',
+    ],
     [
       'zh-CN',
       '向左前方驶入匝道，进入 I-95',
diff --git a/src/i18n/i18n.test.mjs b/src/i18n/i18n.test.mjs
index 1b421f2ff..2a3dcd39b 100644
--- a/src/i18n/i18n.test.mjs
+++ b/src/i18n/i18n.test.mjs
@@ -5,6 +5,7 @@ import {
   applyDocumentTranslations,
   formatDate,
   formatNumber,
+  formatRelativeTime,
   msg,
   persistLocaleAndReload,
   setLocale,
@@ -35,6 +36,9 @@ test('normalizeLocale folds regional variants and rejects unshipped tags', () =>
   assert.equal(normalizeLocale(' RU '), 'ru');
   assert.equal(normalizeLocale('en_GB'), 'en');
   assert.equal(normalizeLocale('xx'), null);
+  assert.equal(normalizeLocale('ja'), 'ja');
+  assert.equal(normalizeLocale('ja-JP'), 'ja');
+  assert.equal(normalizeLocale('JA_jp'), 'ja');
   // Explicit script tags take precedence over regional defaults.
   assert.equal(normalizeLocale('zh'), 'zh-CN');
   assert.equal(normalizeLocale('zh-CN'), 'zh-CN');
@@ -164,7 +168,13 @@ test('Intl formatters follow the active locale', () => {
   assert.equal(formatNumber(1234.5), '1,234.5');
   setLocale('ru');
   assert.equal(formatDate(date, options), '15 января 2026 г.');
+  assert.equal(formatRelativeTime(-5, 'minute'), '5 минут назад');
   assert.equal(formatNumber(1234.5), '1 234,5');
+  // Japanese keeps a number and its unit together, as the catalog does.
+  setLocale('ja');
+  assert.equal(formatDate(date, options), '2026年1月15日');
+  assert.equal(formatRelativeTime(-5, 'minute'), '5分前');
+  assert.equal(formatRelativeTime(-3, 'day'), '3日前');
 });
 
 function fakeElement({ text = '', attributes = {} } = {}) {
diff --git a/src/i18n/index.js b/src/i18n/index.js
index 6fc4becc9..40814f3a3 100644
--- a/src/i18n/index.js
+++ b/src/i18n/index.js
@@ -17,11 +17,18 @@ import {
   writeStoredLocale,
 } from './locale.js';
 import es from './locales/es.json' with { type: 'json' };
+import ja from './locales/ja.json' with { type: 'json' };
 import ru from './locales/ru.json' with { type: 'json' };
 import zhCN from './locales/zh-CN.json' with { type: 'json' };
 import zhHant from './locales/zh-Hant.json' with { type: 'json' };
 
-const CATALOGS = Object.freeze({ es, ru, 'zh-CN': zhCN, 'zh-Hant': zhHant });
+const CATALOGS = Object.freeze({
+  es,
+  ja,
+  ru,
+  'zh-CN': zhCN,
+  'zh-Hant': zhHant,
+});
 
 let currentLocale = DEFAULT_LOCALE;
 
@@ -201,13 +208,19 @@ export function formatUtcDayMonth(value) {
 
 /**
  * Locale-aware relative time (Intl.RelativeTimeFormat), e.g.
- * formatRelativeTime(-5, 'minute') → '5 minutes ago' / '5 минут назад'.
+ * formatRelativeTime(-5, 'minute') → '5 minutes ago' / '5 минут назад' /
+ * '5分前'.
  */
 export function formatRelativeTime(value, unit = 'second', options) {
-  return cachedFormatter('relative', Intl.RelativeTimeFormat, {
+  const text = cachedFormatter('relative', Intl.RelativeTimeFormat, {
     numeric: 'auto',
     ...options,
   }).format(value, unit);
+  // CLDR spaces Japanese (5 分前); the ja catalog writes the number and its
+  // unit together (5分前), so the two must not differ on one screen.
+  return currentLocale === 'ja'
+    ? text.replace(/(?<=\d) (?=\p{Script=Han})/gu, '')
+    : text;
 }
 
 /** Collapse markup whitespace so a message reads the same in HTML and JS. */
diff --git a/src/i18n/locale.js b/src/i18n/locale.js
index 55ed71163..26bd4abc8 100644
--- a/src/i18n/locale.js
+++ b/src/i18n/locale.js
@@ -8,12 +8,20 @@ export const LOCALE_STORAGE_KEY = 'gev:locale:v1';
 export const DEFAULT_LOCALE = 'en';
 
 /** Shipped locales; order does not matter. Adding one: docs/TRANSLATORS.md. */
-export const LOCALES = Object.freeze(['en', 'es', 'ru', 'zh-CN', 'zh-Hant']);
+export const LOCALES = Object.freeze([
+  'en',
+  'es',
+  'ja',
+  'ru',
+  'zh-CN',
+  'zh-Hant',
+]);
 
 /** Each locale named in its own language, for the selector. */
 export const LOCALE_NAMES = Object.freeze({
   en: 'English',
   es: 'Español',
+  ja: '日本語',
   ru: 'Русский',
   'zh-CN': '中文（简体）',
   'zh-Hant': '中文（繁體）',
@@ -23,7 +31,7 @@ export const LOCALE_NAMES = Object.freeze({
 const RTL_LOCALES = Object.freeze([]);
 
 /**
- * Map any language tag to a shipped locale: 'ru-RU' and 'RU' → 'ru',
+ * Map any language tag to a shipped locale: 'ru-RU' and 'RU' → 'ru', 'ja-JP' → 'ja',
  * 'zh', 'zh-CN' and 'zh-Hans-SG' → 'zh-CN';
  * 'zh-Hant', 'zh-TW', 'zh-HK' and 'zh-MO' → 'zh-Hant'.
  * @returns {string|null}
diff --git a/src/layers/directions/index.js b/src/layers/directions/index.js
index b8cceff02..1d5c100ae 100644
--- a/src/layers/directions/index.js
+++ b/src/layers/directions/index.js
@@ -20,7 +20,11 @@ import * as Cesium from 'cesium';
 import { roadLabel } from '../../data/routeSteps.js';
 import { t, msg, formatNumber, translateKnown } from '../../i18n/index.js';
 import { pointerOwnerName } from '../../data/pointerOwnerNames.js';
-import { formatQuantity } from '../../ui/unitLabels.js';
+import {
+  formatQuantity,
+  quantitySeparator,
+  unitLabel,
+} from '../../ui/unitLabels.js';
 
 export const DIRECTIONS_STEP_OVERLAY_SOURCE_ID = 'directions-step';
 export const DIRECTIONS_STEP_OVERLAY_SOURCE_OPTIONS = Object.freeze({
@@ -137,7 +141,9 @@ export function routeDurationText(seconds) {
   if (minutes < 60) return formatQuantity(minutes, 'min');
   const hours = formatQuantity(Math.floor(minutes / 60), 'h');
   const rest = minutes % 60;
-  return rest ? `${hours}\u00a0${formatQuantity(rest, 'min')}` : hours;
+  if (!rest) return hours;
+  // "1 h 5 min"; Japanese runs the parts together: 1時間5分.
+  return `${hours}${quantitySeparator(unitLabel('h'))}${formatQuantity(rest, 'min')}`;
 }
 
 /** "left", "slightly right", … as it reads inside a maneuver sentence. */
diff --git a/src/ui/localePicker.test.mjs b/src/ui/localePicker.test.mjs
index 057642827..8f9f127e2 100644
--- a/src/ui/localePicker.test.mjs
+++ b/src/ui/localePicker.test.mjs
@@ -40,7 +40,7 @@ test('the current language comes first, the rest by their own names', () => {
         nativeNames: LOCALE_NAMES,
       }),
     ),
-    ['ru', 'en', 'es', 'zh-CN', 'zh-Hant'],
+    ['ru', 'en', 'es', 'zh-CN', 'zh-Hant', 'ja'],
   );
   assert.deepEqual(
     codes(
@@ -50,7 +50,7 @@ test('the current language comes first, the rest by their own names', () => {
         nativeNames: LOCALE_NAMES,
       }),
     ),
-    ['es', 'en', 'ru', 'zh-CN', 'zh-Hant'],
+    ['es', 'en', 'ru', 'zh-CN', 'zh-Hant', 'ja'],
   );
   assert.deepEqual(
     codes(
@@ -60,7 +60,7 @@ test('the current language comes first, the rest by their own names', () => {
         nativeNames: LOCALE_NAMES,
       }),
     ),
-    ['en', 'es', 'ru', 'zh-CN', 'zh-Hant'],
+    ['en', 'es', 'ru', 'zh-CN', 'zh-Hant', 'ja'],
   );
   const rows = buildLocaleOptions({
     locales: MANY,
@@ -263,7 +263,7 @@ test('keyboard model: open on the current language, move, choose, close', () =>
   assert.deepEqual(model.keydown({ key: 'ArrowDown' }), { handled: true });
   assert.equal(model.activeOption.code, 'en');
   model.keydown({ key: 'End' });
-  assert.equal(model.activeOption.code, 'zh-Hant');
+  assert.equal(model.activeOption.code, 'ja');
   model.keydown({ key: 'Home' });
   assert.equal(model.activeOption.code, 'ru');
   model.keydown({ key: 'ArrowUp' });
diff --git a/src/ui/unitLabels.js b/src/ui/unitLabels.js
index 1d9bd6a61..3b12da006 100644
--- a/src/ui/unitLabels.js
+++ b/src/ui/unitLabels.js
@@ -1,4 +1,4 @@
-import { formatNumber, msg, t } from '../i18n/index.js';
+import { formatNumber, getLocale, msg, t } from '../i18n/index.js';
 
 // Unit ids stay English in data, params and AI-tool readings; only the label
 // shown on the page is translated. dBZ and °C read the same in every language.
@@ -23,6 +23,17 @@ export function unitLabel(unit) {
   return Object.hasOwn(UNIT_LABELS, unit) ? t(UNIT_LABELS[unit]) : unit || '';
 }
 
+/**
+ * What goes between a number and its unit label: a no-break space, except
+ * before a Japanese unit word, which follows the number directly (5分, 1時間).
+ */
+export function quantitySeparator(label) {
+  return getLocale() === 'ja' &&
+    /^[\p{Script=Han}\p{Script=Hiragana}\p{Script=Katakana}]/u.test(label)
+    ? ''
+    : '\u00a0';
+}
+
 /**
  * A number and its unit that never break apart: "22,0 км/ч". Readings carry
  * no digit grouping ("1008 hPa"), as on weather charts.
@@ -30,7 +41,7 @@ export function unitLabel(unit) {
 export function formatQuantity(value, unit, options) {
   const label = unitLabel(unit);
   const number = formatNumber(value, { useGrouping: false, ...options });
-  return label ? `${number}\u00a0${label}` : number;
+  return label ? `${number}${quantitySeparator(label)}${label}` : number;
 }
 
 /**
diff --git a/src/ui/unitLabels.test.mjs b/src/ui/unitLabels.test.mjs
index c0b1932b1..14b6030e3 100644
--- a/src/ui/unitLabels.test.mjs
+++ b/src/ui/unitLabels.test.mjs
@@ -44,3 +44,14 @@ test('other languages get their decimal separator and no digit grouping', () =>
     setLocale('en');
   }
 });
+
+test('Japanese unit words follow the number directly; symbols keep the space', () => {
+  try {
+    setLocale('ja');
+    assert.equal(formatQuantity(5, 'min'), '5分');
+    assert.equal(formatQuantity(1, 'h'), '1時間');
+    assert.equal(formatQuantity(95, 'kt'), '95\u00a0kt');
+  } finally {
+    setLocale('en');
+  }
+});