diff --git a/.changeset/fix-iframe-client-uat-cookie-domain.md b/.changeset/fix-iframe-client-uat-cookie-domain.md
new file mode 100644
index 00000000000..a7fa09b3670
--- /dev/null
+++ b/.changeset/fix-iframe-client-uat-cookie-domain.md
@@ -0,0 +1,5 @@
+---
+'@clerk/clerk-js': patch
+---
+
+Fix `__client_uat` cookie being set on two different domain scopes when app is loaded in both iframe and non-iframe contexts. `getCookieDomain()` now falls back to `hostname` instead of `undefined` when the eTLD+1 probe fails, and the eTLD+1 probe uses the same `SameSite`/`Secure` attributes as the actual cookie to ensure consistent behavior across contexts.
diff --git a/packages/clerk-js/src/core/auth/__tests__/getCookieDomain.test.ts b/packages/clerk-js/src/core/auth/__tests__/getCookieDomain.test.ts
index 2f8bae19ffb..9de44e8022c 100644
--- a/packages/clerk-js/src/core/auth/__tests__/getCookieDomain.test.ts
+++ b/packages/clerk-js/src/core/auth/__tests__/getCookieDomain.test.ts
@@ -35,6 +35,19 @@ describe('getCookieDomain', () => {
     expect(result).toBe(hostname);
   });
 
+  it('passes cookie attributes to the probe', () => {
+    const hostname = 'app.example.com';
+    const handler: CookieHandler = {
+      get: vi.fn().mockReturnValueOnce(undefined).mockReturnValueOnce('1'),
+      set: vi.fn().mockReturnValue(undefined),
+      remove: vi.fn().mockReturnValue(undefined),
+    };
+    const attrs = { sameSite: 'None', secure: true };
+    getCookieDomain(hostname, handler, attrs);
+    expect(handler.set).toHaveBeenCalledWith('1', { sameSite: 'None', secure: true, domain: 'example.com' });
+    expect(handler.set).toHaveBeenCalledWith('1', { sameSite: 'None', secure: true, domain: 'app.example.com' });
+  });
+
   it('handles localhost', () => {
     const hostname = 'localhost';
     const result = getCookieDomain(hostname);
@@ -53,7 +66,7 @@ describe('getCookieDomain', () => {
     expect(getCookieDomain('bryce-local')).toBe('bryce-local');
   });
 
-  it('returns undefined if the domain could not be determined', () => {
+  it('falls back to hostname if the domain could not be determined', () => {
     const handler: CookieHandler = {
       get: vi.fn().mockReturnValue(undefined),
       set: vi.fn().mockReturnValue(undefined),
@@ -61,7 +74,7 @@ describe('getCookieDomain', () => {
     };
     const hostname = 'app.hello.co.uk';
     const result = getCookieDomain(hostname, handler);
-    expect(result).toBeUndefined();
+    expect(result).toBe(hostname);
   });
 
   it('uses cached value if there is one', () => {
diff --git a/packages/clerk-js/src/core/auth/cookies/clientUat.ts b/packages/clerk-js/src/core/auth/cookies/clientUat.ts
index 7d52af06e43..a90d5a77184 100644
--- a/packages/clerk-js/src/core/auth/cookies/clientUat.ts
+++ b/packages/clerk-js/src/core/auth/cookies/clientUat.ts
@@ -45,7 +45,7 @@ export const createClientUatCookie = (cookieSuffix: string): ClientUatCookieHand
         : 'Strict';
     const secure = getSecureAttribute(sameSite);
     const partitioned = __BUILD_VARIANT_CHIPS__ && secure;
-    const domain = getCookieDomain();
+    const domain = getCookieDomain(undefined, undefined, { sameSite, secure });
 
     // '0' indicates the user is signed out
     let val = '0';
diff --git a/packages/clerk-js/src/core/auth/getCookieDomain.ts b/packages/clerk-js/src/core/auth/getCookieDomain.ts
index 492c2beab12..42a1a4f0a39 100644
--- a/packages/clerk-js/src/core/auth/getCookieDomain.ts
+++ b/packages/clerk-js/src/core/auth/getCookieDomain.ts
@@ -8,7 +8,19 @@ import { createCookieHandler } from '@clerk/shared/cookie';
 let cachedETLDPlusOne: string;
 const eTLDCookie = createCookieHandler('__clerk_test_etld');
 
-export function getCookieDomain(hostname = window.location.hostname, cookieHandler = eTLDCookie) {
+/**
+ * @param hostname - The hostname to determine the eTLD+1 for.
+ * @param cookieHandler - The cookie handler to use for the eTLD+1 probe.
+ * @param cookieAttributes - Optional cookie attributes (sameSite, secure) to use
+ *   during the eTLD+1 probe. These should match the attributes that will be used
+ *   when setting the actual cookie, so the probe accurately reflects whether a
+ *   domain-scoped cookie can be set in the current context.
+ */
+export function getCookieDomain(
+  hostname = window.location.hostname,
+  cookieHandler = eTLDCookie,
+  cookieAttributes?: { sameSite?: string; secure?: boolean },
+) {
   // only compute it once per session to avoid unnecessary cookie ops
   if (cachedETLDPlusOne) {
     return cachedETLDPlusOne;
@@ -28,17 +40,22 @@ export function getCookieDomain(hostname = window.location.hostname, cookieHandl
   // we know for sure that the first entry is definitely a TLD, skip it
   for (let i = hostnameParts.length - 2; i >= 0; i--) {
     const eTLD = hostnameParts.slice(i).join('.');
-    cookieHandler.set('1', { domain: eTLD });
+    cookieHandler.set('1', { ...cookieAttributes, domain: eTLD });
 
     const res = cookieHandler.get();
     if (res === '1') {
-      cookieHandler.remove({ domain: eTLD });
+      cookieHandler.remove({ ...cookieAttributes, domain: eTLD });
       cachedETLDPlusOne = eTLD;
       return eTLD;
     }
 
-    cookieHandler.remove({ domain: eTLD });
+    cookieHandler.remove({ ...cookieAttributes, domain: eTLD });
   }
 
-  return;
+  // Fallback to hostname to ensure domain-scoped cookie rather than host-only.
+  // In restricted contexts (e.g. cross-origin iframes), the set() will silently
+  // fail — which is preferable to creating a host-only cookie that conflicts
+  // with domain-scoped cookies set by non-iframe contexts.
+  cachedETLDPlusOne = hostname;
+  return hostname;
 }