diff --git a/.changeset/orange-bikes-prove.md b/.changeset/orange-bikes-prove.md
new file mode 100644
index 00000000000..0802a6e90e2
--- /dev/null
+++ b/.changeset/orange-bikes-prove.md
@@ -0,0 +1,5 @@
+---
+'@clerk/backend': minor
+---
+
+Trigger a handshake on a signed in, cross origin request to sync session state from a satellite domain.
diff --git a/packages/backend/src/constants.ts b/packages/backend/src/constants.ts
index 08e90a3164f..2a04b3e2bac 100644
--- a/packages/backend/src/constants.ts
+++ b/packages/backend/src/constants.ts
@@ -63,6 +63,7 @@ const Headers = {
   Origin: 'origin',
   Referrer: 'referer',
   SecFetchDest: 'sec-fetch-dest',
+  SecFetchSite: 'sec-fetch-site',
   UserAgent: 'user-agent',
   ReportingEndpoints: 'reporting-endpoints',
 } as const;
diff --git a/packages/backend/src/tokens/__tests__/request.test.ts b/packages/backend/src/tokens/__tests__/request.test.ts
index 8cb17ed8beb..9419688ca90 100644
--- a/packages/backend/src/tokens/__tests__/request.test.ts
+++ b/packages/backend/src/tokens/__tests__/request.test.ts
@@ -1396,4 +1396,188 @@ describe('tokens.authenticateRequest(options)', () => {
       });
     });
   });
+
+  describe('Cross-origin sync', () => {
+    beforeEach(() => {
+      server.use(
+        http.get('https://api.clerk.test/v1/jwks', () => {
+          return HttpResponse.json(mockJwks);
+        }),
+      );
+    });
+
+    test('triggers handshake for cross-origin document request on primary domain', async () => {
+      const request = mockRequestWithCookies(
+        {
+          referer: 'https://satellite.com/signin',
+          'sec-fetch-dest': 'document',
+          origin: 'https://primary.com',
+        },
+        {
+          __session: mockJwt,
+          __client_uat: '12345',
+        },
+        'https://primary.com/dashboard',
+      );
+
+      const requestState = await authenticateRequest(request, {
+        ...mockOptions(),
+        publishableKey: PK_LIVE,
+        domain: 'primary.com',
+        isSatellite: false,
+        signInUrl: 'https://primary.com/sign-in',
+      });
+
+      expect(requestState).toMatchHandshake({
+        reason: AuthErrorReason.PrimaryDomainCrossOriginSync,
+        domain: 'primary.com',
+        signInUrl: 'https://primary.com/sign-in',
+      });
+    });
+
+    test('triggers handshake for cross-site document request on primary domain', async () => {
+      const request = mockRequestWithCookies(
+        {
+          referer: 'https://satellite.com/signin',
+          'sec-fetch-dest': 'document',
+          'sec-fetch-site': 'cross-site',
+          origin: 'https://primary.com',
+        },
+        {
+          __session: mockJwt,
+          __client_uat: '12345',
+        },
+        'https://primary.com/dashboard',
+      );
+
+      const requestState = await authenticateRequest(request, {
+        ...mockOptions(),
+        publishableKey: PK_LIVE,
+        domain: 'primary.com',
+        isSatellite: false,
+        signInUrl: 'https://primary.com/sign-in',
+      });
+
+      expect(requestState).toMatchHandshake({
+        reason: AuthErrorReason.PrimaryDomainCrossOriginSync,
+        domain: 'primary.com',
+        signInUrl: 'https://primary.com/sign-in',
+      });
+    });
+
+    test('does not trigger handshake when referer is same origin', async () => {
+      const request = mockRequestWithCookies(
+        {
+          referer: 'https://primary.com/signin',
+          'sec-fetch-dest': 'document',
+          origin: 'https://primary.com',
+        },
+        {
+          __session: mockJwt,
+          __client_uat: '12345',
+        },
+        'https://primary.com/dashboard',
+      );
+
+      const requestState = await authenticateRequest(request, {
+        ...mockOptions(),
+        publishableKey: PK_LIVE,
+        domain: 'primary.com',
+        isSatellite: false,
+        signInUrl: 'https://primary.com/sign-in',
+      });
+
+      expect(requestState).toBeSignedIn({
+        domain: 'primary.com',
+        isSatellite: false,
+        signInUrl: 'https://primary.com/sign-in',
+      });
+    });
+
+    test('does not trigger handshake when no referer header', async () => {
+      const request = mockRequestWithCookies(
+        {
+          'sec-fetch-dest': 'document',
+          origin: 'https://primary.com',
+        },
+        {
+          __session: mockJwt,
+          __client_uat: '12345',
+        },
+        'https://primary.com/dashboard',
+      );
+
+      const requestState = await authenticateRequest(request, {
+        ...mockOptions(),
+        publishableKey: PK_LIVE,
+        domain: 'primary.com',
+        isSatellite: false,
+        signInUrl: 'https://primary.com/sign-in',
+      });
+
+      expect(requestState).toBeSignedIn({
+        domain: 'primary.com',
+        isSatellite: false,
+        signInUrl: 'https://primary.com/sign-in',
+      });
+    });
+
+    test('does not trigger handshake for non-document requests', async () => {
+      const request = mockRequestWithCookies(
+        {
+          referer: 'https://satellite.com/signin',
+          'sec-fetch-dest': 'empty',
+          origin: 'https://primary.com',
+        },
+        {
+          __session: mockJwt,
+          __client_uat: '12345',
+        },
+        'https://primary.com/api/data',
+      );
+
+      const requestState = await authenticateRequest(request, {
+        ...mockOptions(),
+        publishableKey: PK_LIVE,
+        domain: 'primary.com',
+        isSatellite: false,
+        signInUrl: 'https://primary.com/sign-in',
+      });
+
+      expect(requestState).toBeSignedIn({
+        domain: 'primary.com',
+        isSatellite: false,
+        signInUrl: 'https://primary.com/sign-in',
+      });
+    });
+
+    test('does not trigger handshake when referer header contains invalid URL format', async () => {
+      const request = mockRequestWithCookies(
+        {
+          referer: 'invalid-url-format',
+          'sec-fetch-dest': 'document',
+          origin: 'https://primary.com',
+        },
+        {
+          __session: mockJwt,
+          __client_uat: '12345',
+        },
+        'https://primary.com/dashboard',
+      );
+
+      const requestState = await authenticateRequest(request, {
+        ...mockOptions(),
+        publishableKey: PK_LIVE,
+        domain: 'primary.com',
+        isSatellite: false,
+        signInUrl: 'https://primary.com/sign-in',
+      });
+
+      expect(requestState).toBeSignedIn({
+        domain: 'primary.com',
+        isSatellite: false,
+        signInUrl: 'https://primary.com/sign-in',
+      });
+    });
+  });
 });
diff --git a/packages/backend/src/tokens/authStatus.ts b/packages/backend/src/tokens/authStatus.ts
index d0c60eaecbd..881e5f40ca6 100644
--- a/packages/backend/src/tokens/authStatus.ts
+++ b/packages/backend/src/tokens/authStatus.ts
@@ -107,6 +107,7 @@ export const AuthErrorReason = {
   DevBrowserMissing: 'dev-browser-missing',
   DevBrowserSync: 'dev-browser-sync',
   PrimaryRespondsToSyncing: 'primary-responds-to-syncing',
+  PrimaryDomainCrossOriginSync: 'primary-domain-cross-origin-sync',
   SatelliteCookieNeedsSyncing: 'satellite-needs-syncing',
   SessionTokenAndUATMissing: 'session-token-and-uat-missing',
   SessionTokenMissing: 'session-token-missing',
diff --git a/packages/backend/src/tokens/authenticateContext.ts b/packages/backend/src/tokens/authenticateContext.ts
index cea50da15b0..9bd4fedd335 100644
--- a/packages/backend/src/tokens/authenticateContext.ts
+++ b/packages/backend/src/tokens/authenticateContext.ts
@@ -167,6 +167,30 @@ class AuthenticateContext implements AuthenticateContext {
     return true;
   }
 
+  /**
+   * Determines if the request came from a different origin based on the referrer header.
+   * Used for cross-origin detection in multi-domain authentication flows.
+   *
+   * @returns {boolean} True if referrer exists and is from a different origin, false otherwise.
+   */
+  public isCrossOriginReferrer(): boolean {
+    if (!this.referrer || !this.origin) {
+      return false;
+    }
+
+    try {
+      if (this.getHeader(constants.Headers.SecFetchSite) === 'cross-site') {
+        return true;
+      }
+
+      const referrerOrigin = new URL(this.referrer).origin;
+      return referrerOrigin !== this.origin;
+    } catch {
+      // Invalid referrer URL format
+      return false;
+    }
+  }
+
   private initPublishableKeyValues(options: AuthenticateRequestOptions) {
     assertValidPublishableKey(options.publishableKey);
     this.publishableKey = options.publishableKey;
diff --git a/packages/backend/src/tokens/request.ts b/packages/backend/src/tokens/request.ts
index d212f568432..a8064934a8e 100644
--- a/packages/backend/src/tokens/request.ts
+++ b/packages/backend/src/tokens/request.ts
@@ -553,6 +553,20 @@ export const authenticateRequest: AuthenticateRequest = (async (
         token: authenticateContext.sessionTokenInCookie!,
       });
 
+      // Check for cross-origin requests from satellite domains to primary domain
+      const shouldForceHandshakeForCrossDomain =
+        !authenticateContext.isSatellite && // We're on primary
+        authenticateContext.secFetchDest === 'document' && // Document navigation
+        authenticateContext.isCrossOriginReferrer(); // Came from different domain
+
+      if (shouldForceHandshakeForCrossDomain) {
+        return handleMaybeHandshakeStatus(
+          authenticateContext,
+          AuthErrorReason.PrimaryDomainCrossOriginSync,
+          'Cross-origin request from satellite domain requires handshake',
+        );
+      }
+
       const authObject = signedInRequestState.toAuth();
       // Org sync if necessary
       if (authObject.userId) {