diff --git a/packages/clerk-js/src/core/resources/SignUp.ts b/packages/clerk-js/src/core/resources/SignUp.ts
index bac10783b6e..9c817defffe 100644
--- a/packages/clerk-js/src/core/resources/SignUp.ts
+++ b/packages/clerk-js/src/core/resources/SignUp.ts
@@ -46,6 +46,7 @@ import {
   generateSignatureWithMetamask,
   generateSignatureWithOKXWallet,
   getBaseIdentifier,
+  getBrowserLocale,
   getClerkQueryParam,
   getCoinbaseWalletIdentifier,
   getMetamaskIdentifier,
@@ -95,6 +96,7 @@ export class SignUp extends BaseResource implements SignUpResource {
   createdUserId: string | null = null;
   abandonAt: number | null = null;
   legalAcceptedAt: number | null = null;
+  locale: string | null = null;
 
   /**
    * The current status of the sign-up process.
@@ -154,6 +156,14 @@ export class SignUp extends BaseResource implements SignUpResource {
 
     let finalParams = { ...params };
 
+    // Inject browser locale if not already provided
+    if (!finalParams.locale) {
+      const browserLocale = getBrowserLocale();
+      if (browserLocale) {
+        finalParams.locale = browserLocale;
+      }
+    }
+
     if (!__BUILD_DISABLE_RHC__ && !this.clientBypass() && !this.shouldBypassCaptchaForAttempt(params)) {
       const captchaChallenge = new CaptchaChallenge(SignUp.clerk);
       const captchaParams = await captchaChallenge.managedOrInvisible({ action: 'signup' });
@@ -477,6 +487,7 @@ export class SignUp extends BaseResource implements SignUpResource {
       this.abandonAt = data.abandon_at;
       this.web3wallet = data.web3_wallet;
       this.legalAcceptedAt = data.legal_accepted_at;
+      this.locale = data.locale;
     }
 
     eventBus.emit('resource:update', { resource: this });
@@ -505,6 +516,7 @@ export class SignUp extends BaseResource implements SignUpResource {
       abandon_at: this.abandonAt,
       web3_wallet: this.web3wallet,
       legal_accepted_at: this.legalAcceptedAt,
+      locale: this.locale,
       external_account: this.externalAccount,
       external_account_strategy: this.externalAccount?.strategy,
     };
@@ -620,6 +632,10 @@ class SignUpFuture implements SignUpFutureResource {
     return this.resource.legalAcceptedAt;
   }
 
+  get locale() {
+    return this.resource.locale;
+  }
+
   get unverifiedFields() {
     return this.resource.unverifiedFields;
   }
@@ -677,7 +693,15 @@ class SignUpFuture implements SignUpFutureResource {
 
   async create(params: SignUpFutureCreateParams): Promise<{ error: unknown }> {
     return runAsyncResourceTask(this.resource, async () => {
-      await this._create(params);
+      // Inject browser locale if not already provided
+      const createParams = { ...params };
+      if (!createParams.locale) {
+        const browserLocale = getBrowserLocale();
+        if (browserLocale) {
+          createParams.locale = browserLocale;
+        }
+      }
+      await this._create(createParams);
     });
   }
 
diff --git a/packages/clerk-js/src/test/core-fixtures.ts b/packages/clerk-js/src/test/core-fixtures.ts
index f7d1587fc97..4f7b5ad90d8 100644
--- a/packages/clerk-js/src/test/core-fixtures.ts
+++ b/packages/clerk-js/src/test/core-fixtures.ts
@@ -231,6 +231,8 @@ export const createSignUp = (signUpParams: Partial<SignUpJSON> = {}) => {
     first_name: signUpParams.first_name,
     has_password: signUpParams.has_password,
     last_name: signUpParams.last_name,
+    legal_accepted_at: signUpParams.legal_accepted_at,
+    locale: signUpParams.locale,
     missing_fields: signUpParams.missing_fields,
     object: 'sign_up',
     optional_fields: signUpParams.optional_fields,
diff --git a/packages/clerk-js/src/utils/__tests__/locale.test.ts b/packages/clerk-js/src/utils/__tests__/locale.test.ts
new file mode 100644
index 00000000000..e486aa3f854
--- /dev/null
+++ b/packages/clerk-js/src/utils/__tests__/locale.test.ts
@@ -0,0 +1,33 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+import { getBrowserLocale } from '../locale';
+
+describe('getBrowserLocale()', () => {
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('returns the browser locale when available', () => {
+    vi.stubGlobal('navigator', { language: 'es-ES' });
+
+    expect(getBrowserLocale()).toBe('es-ES');
+  });
+
+  it('returns null as default when navigator.language is not available', () => {
+    vi.stubGlobal('navigator', { language: undefined });
+
+    expect(getBrowserLocale()).toBeNull();
+  });
+
+  it('returns null as default when navigator.language is empty string', () => {
+    vi.stubGlobal('navigator', { language: '' });
+
+    expect(getBrowserLocale()).toBeNull();
+  });
+
+  it('returns null as default when navigator object is not defined', () => {
+    vi.stubGlobal('navigator', undefined);
+
+    expect(getBrowserLocale()).toBeNull();
+  });
+});
diff --git a/packages/clerk-js/src/utils/index.ts b/packages/clerk-js/src/utils/index.ts
index 3fe9714465a..7f12982f36e 100644
--- a/packages/clerk-js/src/utils/index.ts
+++ b/packages/clerk-js/src/utils/index.ts
@@ -23,6 +23,7 @@ export * from './props';
 export * from './queryStateParams';
 export * from './querystring';
 export * from './runtime';
+export * from './locale';
 export * from './url';
 export * from './web3';
 export * from './windowNavigate';
diff --git a/packages/clerk-js/src/utils/locale.ts b/packages/clerk-js/src/utils/locale.ts
new file mode 100644
index 00000000000..63be37cd018
--- /dev/null
+++ b/packages/clerk-js/src/utils/locale.ts
@@ -0,0 +1,25 @@
+import { inBrowser } from '@clerk/shared/browser';
+
+const DEFAULT_LOCALE = null;
+
+/**
+ * Detects the user's preferred locale from the browser.
+ * Falls back to null if locale cannot be determined.
+ *
+ * @returns The browser's reported locale string (typically BCP 47 format like 'en-US', 'es-ES') or null if locale cannot be determined.
+ */
+export function getBrowserLocale(): string | null {
+  if (!inBrowser()) {
+    return DEFAULT_LOCALE;
+  }
+
+  // Get locale from the browser
+  const locale = navigator?.language;
+
+  // Validate that we got a non-empty string
+  if (!locale || typeof locale !== 'string' || locale.trim() === '') {
+    return DEFAULT_LOCALE;
+  }
+
+  return locale;
+}
diff --git a/packages/react/src/stateProxy.ts b/packages/react/src/stateProxy.ts
index 995c78dc647..ab65bcf8912 100644
--- a/packages/react/src/stateProxy.ts
+++ b/packages/react/src/stateProxy.ts
@@ -194,6 +194,9 @@ export class StateProxy implements State {
         get legalAcceptedAt() {
           return gateProperty(target, 'legalAcceptedAt', null);
         },
+        get locale() {
+          return gateProperty(target, 'locale', null);
+        },
         get status() {
           return gateProperty(target, 'status', 'missing_requirements');
         },
diff --git a/packages/types/src/json.ts b/packages/types/src/json.ts
index 9ab16c1540c..32638694c30 100644
--- a/packages/types/src/json.ts
+++ b/packages/types/src/json.ts
@@ -136,6 +136,7 @@ export interface SignUpJSON extends ClerkResourceJSON {
   created_user_id: string | null;
   abandon_at: number | null;
   legal_accepted_at: number | null;
+  locale: string | null;
   verifications: SignUpVerificationsJSON | null;
 }
 
diff --git a/packages/types/src/signUp.ts b/packages/types/src/signUp.ts
index 63976a78fa1..f6c9e9bd48b 100644
--- a/packages/types/src/signUp.ts
+++ b/packages/types/src/signUp.ts
@@ -60,6 +60,7 @@ export interface SignUpResource extends ClerkResource {
   createdUserId: string | null;
   abandonAt: number | null;
   legalAcceptedAt: number | null;
+  locale: string | null;
 
   create: (params: SignUpCreateParams) => Promise<SignUpResource>;
 
diff --git a/packages/types/src/signUpCommon.ts b/packages/types/src/signUpCommon.ts
index 573b38d6341..62b81715a2a 100644
--- a/packages/types/src/signUpCommon.ts
+++ b/packages/types/src/signUpCommon.ts
@@ -100,6 +100,7 @@ export type SignUpCreateParams = Partial<
     oidcPrompt: string;
     oidcLoginHint: string;
     channel: PhoneCodeChannel;
+    locale?: string;
   } & Omit<SnakeToCamel<Record<SignUpAttributeField | SignUpVerifiableField, string>>, 'legalAccepted'>
 >;
 
diff --git a/packages/types/src/signUpFuture.ts b/packages/types/src/signUpFuture.ts
index 660d5622f5f..a9c180245b5 100644
--- a/packages/types/src/signUpFuture.ts
+++ b/packages/types/src/signUpFuture.ts
@@ -8,6 +8,7 @@ interface SignUpFutureAdditionalParams {
   lastName?: string;
   unsafeMetadata?: SignUpUnsafeMetadata;
   legalAccepted?: boolean;
+  locale?: string;
 }
 
 export interface SignUpFutureCreateParams extends SignUpFutureAdditionalParams {
@@ -136,6 +137,8 @@ export interface SignUpFutureResource {
 
   readonly legalAcceptedAt: number | null;
 
+  readonly locale: string | null;
+
   create: (params: SignUpFutureCreateParams) => Promise<{ error: unknown }>;
 
   update: (params: SignUpFutureUpdateParams) => Promise<{ error: unknown }>;
diff --git a/playground/app-router/.gitignore b/playground/app-router/.gitignore
index 8f322f0d8f4..927a115dba8 100644
--- a/playground/app-router/.gitignore
+++ b/playground/app-router/.gitignore
@@ -25,7 +25,7 @@ yarn-debug.log*
 yarn-error.log*
 
 # local env files
-.env*.local
+.env*
 
 # vercel
 .vercel
@@ -33,3 +33,6 @@ yarn-error.log*
 # typescript
 *.tsbuildinfo
 next-env.d.ts
+
+# clerk configuration (can include secrets)
+/.clerk/
diff --git a/playground/app-router/README.md b/playground/app-router/README.md
index cde9c0cde80..293d5ade579 100644
--- a/playground/app-router/README.md
+++ b/playground/app-router/README.md
@@ -12,11 +12,11 @@ yarn dev
 pnpm dev
 ```
 
-Open [http://localhost:3000](http://localhost:3000) with your browser to see the result.
+Open [http://localhost:4011](http://localhost:4011) with your browser to see the result.
 
 You can start editing the page by modifying `app/page.tsx`. The page auto-updates as you edit the file.
 
-[http://localhost:3000/api/hello](http://localhost:3000/api/hello) is an endpoint that uses [Route Handlers](https://nextjs.org/docs/routing/route-handlers). This endpoint can be edited in `app/api/hello/route.ts`.
+[http://localhost:4011/api/hello](http://localhost:4011/api/hello) is an endpoint that uses [Route Handlers](https://nextjs.org/docs/routing/route-handlers). This endpoint can be edited in `app/api/hello/route.ts`.
 
 This project uses [`next/font`](https://nextjs.org/docs/basic-features/font-optimization) to automatically optimize and load Inter, a custom Google Font.