diff --git a/src/SdkConfig.ts b/src/SdkConfig.ts
index e8ac0dcee34..f0bfafab9fd 100644
--- a/src/SdkConfig.ts
+++ b/src/SdkConfig.ts
@@ -29,6 +29,8 @@ export interface ConfigOptions {
     // sso_immediate_redirect is deprecated in favour of sso_redirect_options.immediate
     sso_immediate_redirect?: boolean;
     sso_redirect_options?: ISsoRedirectOptions;
+
+    custom_translations_url?: string;
 }
 /* eslint-enable camelcase*/
 
diff --git a/src/languageHandler.tsx b/src/languageHandler.tsx
index e9991dcad0e..a2037e640f6 100644
--- a/src/languageHandler.tsx
+++ b/src/languageHandler.tsx
@@ -1,8 +1,8 @@
 /*
 Copyright 2017 MTRNord and Cooperative EITA
 Copyright 2017 Vector Creations Ltd.
-Copyright 2019, 2020 The Matrix.org Foundation C.I.C.
 Copyright 2019 Michael Telatynski <7t3chguy@gmail.com>
+Copyright 2019 - 2022 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -21,11 +21,13 @@ import request from 'browser-request';
 import counterpart from 'counterpart';
 import React from 'react';
 import { logger } from "matrix-js-sdk/src/logger";
+import { Optional } from "matrix-events-sdk";
 
 import SettingsStore from "./settings/SettingsStore";
 import PlatformPeg from "./PlatformPeg";
 import { SettingLevel } from "./settings/SettingLevel";
 import { retry } from "./utils/promise";
+import SdkConfig from "./SdkConfig";
 
 // @ts-ignore - $webapp is a webpack resolve alias pointing to the output directory, see webpack config
 import webpackLangJsonUrl from "$webapp/i18n/languages.json";
@@ -394,10 +396,11 @@ export function setLanguage(preferredLangs: string | string[]) {
         }
 
         return getLanguageRetry(i18nFolder + availLangs[langToUse].fileName);
-    }).then((langData) => {
+    }).then(async (langData) => {
         counterpart.registerTranslations(langToUse, langData);
+        await registerCustomTranslations();
         counterpart.setLocale(langToUse);
-        SettingsStore.setValue("language", null, SettingLevel.DEVICE, langToUse);
+        await SettingsStore.setValue("language", null, SettingLevel.DEVICE, langToUse);
         // Adds a lot of noise to test runs, so disable logging there.
         if (process.env.NODE_ENV !== "test") {
             logger.log("set language to " + langToUse);
@@ -407,8 +410,9 @@ export function setLanguage(preferredLangs: string | string[]) {
         if (langToUse !== "en") {
             return getLanguageRetry(i18nFolder + availLangs['en'].fileName);
         }
-    }).then((langData) => {
+    }).then(async (langData) => {
         if (langData) counterpart.registerTranslations('en', langData);
+        await registerCustomTranslations();
     });
 }
 
@@ -581,3 +585,83 @@ function getLanguage(langPath: string): Promise<object> {
         );
     });
 }
+
+export interface ICustomTranslations {
+    // Format is a map of english string to language to override
+    [str: string]: {
+        [lang: string]: string;
+    };
+}
+
+let cachedCustomTranslations: Optional<ICustomTranslations> = null;
+let cachedCustomTranslationsExpire = 0; // zero to trigger expiration right away
+
+// This awkward class exists so the test runner can get at the function. It is
+// not intended for practical or realistic usage.
+export class CustomTranslationOptions {
+    public static lookupFn: (url: string) => ICustomTranslations;
+
+    private constructor() {
+        // static access for tests only
+    }
+}
+
+/**
+ * If a custom translations file is configured, it will be parsed and registered.
+ * If no customization is made, or the file can't be parsed, no action will be
+ * taken.
+ *
+ * This function should be called *after* registering other translations data to
+ * ensure it overrides strings properly.
+ */
+export async function registerCustomTranslations() {
+    const lookupUrl = SdkConfig.get().custom_translations_url;
+    if (!lookupUrl) return; // easy - nothing to do
+
+    try {
+        let json: ICustomTranslations;
+        if (Date.now() >= cachedCustomTranslationsExpire) {
+            json = CustomTranslationOptions.lookupFn
+                ? CustomTranslationOptions.lookupFn(lookupUrl)
+                : (await (await fetch(lookupUrl)).json() as ICustomTranslations);
+            cachedCustomTranslations = json;
+
+            // Set expiration to the future, but not too far. Just trying to avoid
+            // repeated, successive, calls to the server rather than anything long-term.
+            cachedCustomTranslationsExpire = Date.now() + (5 * 60 * 1000);
+        } else {
+            json = cachedCustomTranslations;
+        }
+
+        // If the (potentially cached) json is invalid, don't use it.
+        if (!json) return;
+
+        // We convert the operator-friendly version into something counterpart can
+        // consume.
+        const langs: {
+            // same structure, just flipped key order
+            [lang: string]: {
+                [str: string]: string;
+            };
+        } = {};
+        for (const [str, translations] of Object.entries(json)) {
+            for (const [lang, newStr] of Object.entries(translations)) {
+                if (!langs[lang]) langs[lang] = {};
+                langs[lang][str] = newStr;
+            }
+        }
+
+        // Finally, tell counterpart about our translations
+        for (const [lang, translations] of Object.entries(langs)) {
+            counterpart.registerTranslations(lang, translations);
+        }
+    } catch (e) {
+        // We consume all exceptions because it's considered non-fatal for custom
+        // translations to break. Most failures will be during initial development
+        // of the json file and not (hopefully) at runtime.
+        logger.warn("Ignoring error while registering custom translations: ", e);
+
+        // Like above: trigger a cache of the json to avoid successive calls.
+        cachedCustomTranslationsExpire = Date.now() + (5 * 60 * 1000);
+    }
+}
diff --git a/test/languageHandler-test.ts b/test/languageHandler-test.ts
new file mode 100644
index 00000000000..38a9db7be92
--- /dev/null
+++ b/test/languageHandler-test.ts
@@ -0,0 +1,70 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import SdkConfig from "../src/SdkConfig";
+import {
+    _t,
+    CustomTranslationOptions,
+    ICustomTranslations,
+    registerCustomTranslations,
+    setLanguage,
+} from "../src/languageHandler";
+
+describe('languageHandler', () => {
+    afterEach(() => {
+        SdkConfig.unset();
+        CustomTranslationOptions.lookupFn = undefined;
+    });
+
+    it('should support overriding translations', async () => {
+        const str = "This is a test string that does not exist in the app.";
+        const enOverride = "This is the English version of a custom string.";
+        const deOverride = "This is the German version of a custom string.";
+        const overrides: ICustomTranslations = {
+            [str]: {
+                "en": enOverride,
+                "de": deOverride,
+            },
+        };
+
+        const lookupUrl = "/translations.json";
+        const fn = (url: string): ICustomTranslations => {
+            expect(url).toEqual(lookupUrl);
+            return overrides;
+        };
+
+        // First test that overrides aren't being used
+
+        await setLanguage("en");
+        expect(_t(str)).toEqual(str);
+
+        await setLanguage("de");
+        expect(_t(str)).toEqual(str);
+
+        // Now test that they *are* being used
+        SdkConfig.add({
+            custom_translations_url: lookupUrl,
+        });
+        CustomTranslationOptions.lookupFn = fn;
+        await registerCustomTranslations();
+
+        await setLanguage("en");
+        expect(_t(str)).toEqual(enOverride);
+
+        await setLanguage("de");
+        expect(_t(str)).toEqual(deOverride);
+    });
+});