diff --git a/packages/client/src/client/authExtensions.ts b/packages/client/src/client/authExtensions.ts
index 80efc0a12..cb476c12f 100644
--- a/packages/client/src/client/authExtensions.ts
+++ b/packages/client/src/client/authExtensions.ts
@@ -233,6 +233,16 @@ export interface PrivateKeyJwtProviderOptions {
      * Space-separated scopes values requested by the client.
      */
     scope?: string;
+
+    /**
+     * Optional custom claims to include in the JWT assertion.
+     * These are merged with the standard claims (`iss`, `sub`, `aud`, `exp`, `iat`, `jti`),
+     * with custom claims taking precedence for any overlapping keys.
+     *
+     * Useful for including additional claims that help scope the access token
+     * with finer granularity than what scopes alone allow.
+     */
+    claims?: Record<string, unknown>;
 }
 
 /**
@@ -277,7 +287,8 @@ export class PrivateKeyJwtProvider implements OAuthClientProvider {
             subject: options.clientId,
             privateKey: options.privateKey,
             alg: options.algorithm,
-            lifetimeSeconds: options.jwtLifetimeSeconds
+            lifetimeSeconds: options.jwtLifetimeSeconds,
+            claims: options.claims
         });
     }
 
diff --git a/packages/client/test/client/authExtensions.test.ts b/packages/client/test/client/authExtensions.test.ts
index f7737077e..16c3ea33e 100644
--- a/packages/client/test/client/authExtensions.test.ts
+++ b/packages/client/test/client/authExtensions.test.ts
@@ -424,6 +424,49 @@ describe('createPrivateKeyJwtAuth', () => {
             /Key for the RS256 algorithm must be one of type CryptoKey, KeyObject, or JSON Web Key/
         );
     });
+
+    it('includes custom claims in the signed JWT assertion', async () => {
+        const addClientAuth = createPrivateKeyJwtAuth({
+            issuer: 'client-id',
+            subject: 'client-id',
+            privateKey: 'a-string-secret-at-least-256-bits-long',
+            alg: 'HS256',
+            claims: { tenant_id: 'org-123', role: 'admin' }
+        });
+
+        const params = new URLSearchParams();
+        await addClientAuth(new Headers(), params, 'https://auth.example.com/token', undefined);
+
+        const assertion = params.get('client_assertion');
+        expect(assertion).toBeTruthy();
+
+        const jose = await import('jose');
+        const decoded = jose.decodeJwt(assertion!);
+        expect(decoded.tenant_id).toBe('org-123');
+        expect(decoded.role).toBe('admin');
+        expect(decoded.iss).toBe('client-id');
+        expect(decoded.sub).toBe('client-id');
+    });
+
+    it('passes custom claims through PrivateKeyJwtProvider', async () => {
+        const provider = new PrivateKeyJwtProvider({
+            clientId: 'client-id',
+            privateKey: 'a-string-secret-at-least-256-bits-long',
+            algorithm: 'HS256',
+            claims: { tenant_id: 'org-456' }
+        });
+
+        const params = new URLSearchParams();
+        await provider.addClientAuthentication(new Headers(), params, 'https://auth.example.com/token', undefined);
+
+        const assertion = params.get('client_assertion');
+        expect(assertion).toBeTruthy();
+
+        const jose = await import('jose');
+        const decoded = jose.decodeJwt(assertion!);
+        expect(decoded.tenant_id).toBe('org-456');
+        expect(decoded.iss).toBe('client-id');
+    });
 });
 
 describe('CrossAppAccessProvider', () => {