diff --git a/airflow/www/extensions/init_appbuilder.py b/airflow/www/extensions/init_appbuilder.py
index a3b48cc8d9d61..2238066e6e953 100644
--- a/airflow/www/extensions/init_appbuilder.py
+++ b/airflow/www/extensions/init_appbuilder.py
@@ -212,6 +212,10 @@ def init_app(self, app, session):
         auth_manager = create_auth_manager()
         auth_manager.appbuilder = self
         auth_manager.init()
+        # FAB auth manager is used in both the old FAB UI and the new React UI backend by Fastapi.
+        # It can behave differently depending on the application. Setting this flag so that it knows this
+        # instance is run in FAB.
+        auth_manager.is_in_fab = True
         if hasattr(auth_manager, "security_manager"):
             self.sm = auth_manager.security_manager
         else:
diff --git a/providers/fab/src/airflow/providers/fab/auth_manager/fab_auth_manager.py b/providers/fab/src/airflow/providers/fab/auth_manager/fab_auth_manager.py
index c3d086b56e168..698f2a9219f63 100644
--- a/providers/fab/src/airflow/providers/fab/auth_manager/fab_auth_manager.py
+++ b/providers/fab/src/airflow/providers/fab/auth_manager/fab_auth_manager.py
@@ -146,11 +146,21 @@ class FabAuthManager(BaseAuthManager[User]):
 
     appbuilder: AirflowAppBuilder | None = None
 
+    is_in_fab: bool = False
+    """
+    Whether the instance is run in FAB or Fastapi.
+    Can be deleted once the Airflow 2 legacy UI is removed.
+    """
+
     def init(self) -> None:
         """Run operations when Airflow is initializing."""
         if self.appbuilder:
             self._sync_appbuilder_roles()
 
+    @cached_property
+    def fastapi_endpoint(self) -> str:
+        return conf.get("fastapi", "base_url")
+
     @staticmethod
     def get_cli_commands() -> list[CLICommand]:
         """Vends CLI commands to be included in Airflow CLI."""
@@ -443,12 +453,15 @@ def security_manager(self) -> FabAirflowSecurityManagerOverride:
 
     def get_url_login(self, **kwargs) -> str:
         """Return the login page url."""
-        if not self.security_manager.auth_view:
-            raise AirflowException("`auth_view` not defined in the security manager.")
-        if next_url := kwargs.get("next_url"):
-            return url_for(f"{self.security_manager.auth_view.endpoint}.login", next=next_url)
+        if self.is_in_fab:
+            if not self.security_manager.auth_view:
+                raise AirflowException("`auth_view` not defined in the security manager.")
+            if next_url := kwargs.get("next_url"):
+                return url_for(f"{self.security_manager.auth_view.endpoint}.login", next=next_url)
+            else:
+                return url_for(f"{self.security_manager.auth_view.endpoint}.login")
         else:
-            return url_for(f"{self.security_manager.auth_view.endpoint}.login")
+            return f"{self.fastapi_endpoint}/auth/login"
 
     def get_url_logout(self):
         """Return the logout page url."""
diff --git a/providers/fab/tests/provider_tests/fab/auth_manager/test_fab_auth_manager.py b/providers/fab/tests/provider_tests/fab/auth_manager/test_fab_auth_manager.py
index 1fef2598ba0e4..a6291f20b572d 100644
--- a/providers/fab/tests/provider_tests/fab/auth_manager/test_fab_auth_manager.py
+++ b/providers/fab/tests/provider_tests/fab/auth_manager/test_fab_auth_manager.py
@@ -574,26 +574,9 @@ class TestSecurityManager:
         ):
             auth_manager_with_appbuilder.security_manager
 
-    @pytest.mark.db_test
-    def test_get_url_login_when_auth_view_not_defined(self, auth_manager_with_appbuilder):
-        with pytest.raises(AirflowException, match="`auth_view` not defined in the security manager."):
-            auth_manager_with_appbuilder.get_url_login()
-
-    @pytest.mark.db_test
-    @mock.patch("airflow.providers.fab.auth_manager.fab_auth_manager.url_for")
-    def test_get_url_login(self, mock_url_for, auth_manager_with_appbuilder):
-        auth_manager_with_appbuilder.security_manager.auth_view = Mock()
-        auth_manager_with_appbuilder.security_manager.auth_view.endpoint = "test_endpoint"
-        auth_manager_with_appbuilder.get_url_login()
-        mock_url_for.assert_called_once_with("test_endpoint.login")
-
-    @pytest.mark.db_test
-    @mock.patch("airflow.providers.fab.auth_manager.fab_auth_manager.url_for")
-    def test_get_url_login_with_next(self, mock_url_for, auth_manager_with_appbuilder):
-        auth_manager_with_appbuilder.security_manager.auth_view = Mock()
-        auth_manager_with_appbuilder.security_manager.auth_view.endpoint = "test_endpoint"
-        auth_manager_with_appbuilder.get_url_login(next_url="next_url")
-        mock_url_for.assert_called_once_with("test_endpoint.login", next="next_url")
+    def test_get_url_login(self, auth_manager):
+        result = auth_manager.get_url_login()
+        assert result == "http://localhost:29091/auth/login"
 
     @pytest.mark.db_test
     def test_get_url_logout_when_auth_view_not_defined(self, auth_manager_with_appbuilder):