diff --git a/packages/google-auth/google/auth/impersonated_credentials.py b/packages/google-auth/google/auth/impersonated_credentials.py
index 2838e138ff92..7239a37b4ec6 100644
--- a/packages/google-auth/google/auth/impersonated_credentials.py
+++ b/packages/google-auth/google/auth/impersonated_credentials.py
@@ -362,8 +362,12 @@ def _build_regional_access_boundary_lookup_url(
 
         Returns:
             Optional[str]: The URL for the Regional Access Boundary lookup endpoint, or None
-                 if the service account email is missing.
+                 if the service account email is missing. Returns None if the subject is populated.
         """
+        if self._subject:
+            # RAB does not apply to Workspace User Accounts via Domain-wide Delegation.
+            return None
+
         if not self.service_account_email:
             _LOGGER.error(
                 "Service account email is required to build the Regional Access Boundary lookup URL for impersonated credentials."
diff --git a/packages/google-auth/google/auth/jwt.py b/packages/google-auth/google/auth/jwt.py
index 1241aee70121..754079eb796f 100644
--- a/packages/google-auth/google/auth/jwt.py
+++ b/packages/google-auth/google/auth/jwt.py
@@ -585,7 +585,17 @@ def _perform_refresh_token(self, request):
         self.token, self.expiry = self._make_jwt()
 
     def _build_regional_access_boundary_lookup_url(self, request=None):
-        """Builds the lookup URL using the service account's email address."""
+        """Builds the lookup URL using the service account's email address.
+
+        Returns None if the subject is populated.
+        """
+        # In jwt.Credentials, subject defaults to client_email (which is the issuer).
+        # We must check self._subject != self._issuer to correctly determine if
+        # Domain-Wide Delegation is active.
+        if self._subject and self._subject != self._issuer:
+            # RAB does not apply to Workspace User Accounts via Domain-wide Delegation.
+            return None
+
         if not self.signer_email:
             return None
 
diff --git a/packages/google-auth/google/oauth2/service_account.py b/packages/google-auth/google/oauth2/service_account.py
index 7f719ade2cdb..0abc78b5000d 100644
--- a/packages/google-auth/google/oauth2/service_account.py
+++ b/packages/google-auth/google/oauth2/service_account.py
@@ -514,8 +514,12 @@ def _build_regional_access_boundary_lookup_url(
 
         Returns:
             Optional[str]: The URL for the Regional Access Boundary lookup endpoint, or None
-                 if the service account email is missing.
+                 if the service account email is missing. Returns None if the subject is populated.
         """
+        if self._subject:
+            # RAB does not apply to Workspace User Accounts via Domain-wide Delegation.
+            return None
+
         if not self.service_account_email:
             _LOGGER.error(
                 "Service account email is required to build the Regional Access Boundary lookup URL for service account credentials."
diff --git a/packages/google-auth/tests/oauth2/test_service_account.py b/packages/google-auth/tests/oauth2/test_service_account.py
index 1d70543057d7..9573aad7e970 100644
--- a/packages/google-auth/tests/oauth2/test_service_account.py
+++ b/packages/google-auth/tests/oauth2/test_service_account.py
@@ -290,6 +290,10 @@ def test_build_regional_access_boundary_lookup_url_mtls(self, monkeypatch):
         )
         assert url == expected_url
 
+    def test_build_regional_access_boundary_lookup_url_with_subject(self):
+        credentials = self.make_credentials().with_subject("user@example.com")
+        assert credentials._build_regional_access_boundary_lookup_url() is None
+
     def test_with_token_uri(self):
         credentials = self.make_credentials()
         new_token_uri = "https://example2.com/oauth2/token"
diff --git a/packages/google-auth/tests/test_impersonated_credentials.py b/packages/google-auth/tests/test_impersonated_credentials.py
index 1207ed874b31..215c68620aa0 100644
--- a/packages/google-auth/tests/test_impersonated_credentials.py
+++ b/packages/google-auth/tests/test_impersonated_credentials.py
@@ -759,6 +759,10 @@ def test_build_regional_access_boundary_lookup_url_success_mtls(self, monkeypatc
         )
         assert url == expected_url
 
+    def test_build_regional_access_boundary_lookup_url_with_subject(self):
+        credentials = self.make_credentials(subject="user@example.com")
+        assert credentials._build_regional_access_boundary_lookup_url() is None
+
     def test_with_scopes_provide_default_scopes(self):
         credentials = self.make_credentials()
         credentials._target_scopes = []
diff --git a/packages/google-auth/tests/test_jwt.py b/packages/google-auth/tests/test_jwt.py
index 27b951b8b7bc..8b90b0ecd093 100644
--- a/packages/google-auth/tests/test_jwt.py
+++ b/packages/google-auth/tests/test_jwt.py
@@ -559,7 +559,14 @@ def test_build_regional_access_boundary_lookup_url_standard(self, monkeypatch):
         # Mock check_use_client_cert to return False to simulate standard TLS
         monkeypatch.setattr(_mtls_helper, "check_use_client_cert", lambda: False)
 
-        url = self.credentials._build_regional_access_boundary_lookup_url()
+        credentials = jwt.Credentials(
+            self.credentials._signer,
+            self.credentials.signer_email,
+            self.credentials.signer_email,
+            self.credentials._audience,
+        )
+
+        url = credentials._build_regional_access_boundary_lookup_url()
         expected_url = "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/{}/allowedLocations".format(
             self.SERVICE_ACCOUNT_EMAIL
         )
@@ -571,12 +578,28 @@ def test_build_regional_access_boundary_lookup_url_mtls(self, monkeypatch):
         # Mock check_use_client_cert to return True to simulate mTLS
         monkeypatch.setattr(_mtls_helper, "check_use_client_cert", lambda: True)
 
-        url = self.credentials._build_regional_access_boundary_lookup_url()
+        credentials = jwt.Credentials(
+            self.credentials._signer,
+            self.credentials.signer_email,
+            self.credentials.signer_email,
+            self.credentials._audience,
+        )
+
+        url = credentials._build_regional_access_boundary_lookup_url()
         expected_url = "https://iamcredentials.mtls.googleapis.com/v1/projects/-/serviceAccounts/{}/allowedLocations".format(
             self.SERVICE_ACCOUNT_EMAIL
         )
         assert url == expected_url
 
+    def test_build_regional_access_boundary_lookup_url_with_subject(self):
+        credentials = jwt.Credentials(
+            self.credentials._signer,
+            self.credentials._issuer,
+            "user@example.com",
+            self.credentials._audience,
+        )
+        assert credentials._build_regional_access_boundary_lookup_url() is None
+
     def test_cloning_retains_rab_manager_data(self):
         self.credentials._rab_manager._data = mock.sentinel.rab_data