@@ -31,7 +31,10 @@
 import firebase_functions.core as _core
 import firebase_functions.options as _options
 import firebase_functions.private.util as _util
-from firebase_functions.options import AuthOptions
+from firebase_functions.options import (
+    IS_NOT_TENANT,  # noqa: F401
+    AuthOptions,
+)
 
 
 @_dataclasses.dataclass(frozen=True)
@@ -506,6 +509,8 @@ def on_user_created(
 ) -> _typing.Callable[[AuthEventCallable], AuthEventCallable]:
     """
     Handles an event that is triggered when a user is created in Firebase Authentication.
+    Pass ``tenant_id=identity_fn.IS_NOT_TENANT`` to only trigger on users in the
+    default project (not scoped to any Identity Platform tenant).
 
     Example:
 
@@ -533,7 +538,7 @@ def on_user_created_decorator(func: AuthEventCallable):
 
         @_functools.wraps(func)
         def on_user_created_wrapped(raw: _ce.CloudEvent):
-            return user_event_handler(func, raw)
+            return user_event_handler(func, raw, options)
 
         _util.set_func_endpoint_attr(
             on_user_created_wrapped,
@@ -553,6 +558,8 @@ def on_user_deleted(
 ) -> _typing.Callable[[AuthEventCallable], AuthEventCallable]:
     """
     Handles an event that is triggered when a user is deleted in Firebase Authentication.
+    Pass ``tenant_id=identity_fn.IS_NOT_TENANT`` to only trigger on users in the
+    default project (not scoped to any Identity Platform tenant).
 
     Example:
 
@@ -580,7 +587,7 @@ def on_user_deleted_decorator(func: AuthEventCallable):
 
         @_functools.wraps(func)
         def on_user_deleted_wrapped(raw: _ce.CloudEvent):
-            return user_event_handler(func, raw)
+            return user_event_handler(func, raw, options)
 
         _util.set_func_endpoint_attr(
             on_user_deleted_wrapped,
 
@@ -37,6 +37,14 @@
 """Special configuration value to reset configuration to platform default."""
 
 
+class Sentinel(_util.Sentinel):
+    """Sentinel class for special option configuration values."""
+
+
+IS_NOT_TENANT = Sentinel("Constant to represent the absence of a tenant ID in Auth event triggers.")
+"""Constant to represent the absence of a tenant ID in Auth event triggers."""
+
+
 class VpcEgressSetting(str, _enum.Enum):
     """Valid settings for VPC egress."""
 
@@ -1062,9 +1070,12 @@ class AuthOptions(EventHandlerOptions):
     Internal use only.
     """
 
-    tenant_id: str | Expression[str] | None = None
+    tenant_id: str | Expression[str] | Sentinel | None = None
     """
     An optional tenant ID to scope the trigger to.
+    If not set, the function triggers on users across all tenants.
+    Set to ``IS_NOT_TENANT`` to only trigger on users in the default
+    project (no tenant).
     """
 
     def _endpoint(
@@ -1073,9 +1084,21 @@ def _endpoint(
     ) -> _manifest.ManifestEndpoint:
         assert kwargs["event_type"] is not None
         event_filters: dict[str, str | Expression[str]] = {}
-        if self.tenant_id is not None:
+        # Eventarc does not support empty-string filters to match "no tenant". When
+        # tenant_id is IS_NOT_TENANT (a Sentinel), we omit the Eventarc filter so all
+        # events are delivered and filter out tenant-scoped events at runtime instead.
+        if isinstance(self.tenant_id, _util.Sentinel):
+            if self.tenant_id != IS_NOT_TENANT:
+                raise ValueError(
+                    f"Invalid sentinel for tenant_id: {self.tenant_id.description!r}. "
+                    "Only IS_NOT_TENANT is supported."
+                )
+        elif self.tenant_id is not None:
             if self.tenant_id == "":
-                raise ValueError("AuthOptions: tenant_id cannot be an empty string.")
+                raise ValueError(
+                    "AuthOptions: tenant_id cannot be an empty string; "
+                    "use IS_NOT_TENANT to match users without a tenant."
+                )
             event_filters["tenantid"] = self.tenant_id
 
         endpoint = super()._endpoint(
 
@@ -35,6 +35,7 @@
 )
 from functions_framework import logging as _logging
 
+import firebase_functions.options as _options
 import firebase_functions.private.token_verifier as _token_verifier
 import firebase_functions.private.util as _util
 from firebase_functions.core import _with_init
@@ -509,10 +510,20 @@ def _auth_event_from_ce(raw: _ce.CloudEvent):
 def user_event_handler(
     func: _typing.Callable,
     raw: _ce.CloudEvent,
+    options: _options.AuthOptions | None = None,
 ) -> None:
     """
     Runtime handler invoked by functions-framework for v2 Auth triggers
     (on_user_created and on_user_deleted).
     """
+    # Eventarc cannot filter for the absence of a tenant attribute, so when
+    # tenant_id=IS_NOT_TENANT we drop events belonging to any tenant at runtime
+    # before unpacking the CloudEvent payload.
+    if (
+        options is not None
+        and options.tenant_id == _options.IS_NOT_TENANT
+        and raw._get_attributes().get("tenantid") is not None
+    ):
+        return
     event = _auth_event_from_ce(raw)
     _with_init(func)(event)
@@ -11,7 +11,7 @@
 from flask import Flask, Request
 from werkzeug.test import EnvironBuilder
 
-from firebase_functions import core, identity_fn, params
+from firebase_functions import core, identity_fn, options, params
 from firebase_functions.private import _identity_fn, serving
 
 token_verifier_mock = MagicMock()
@@ -356,3 +356,101 @@ def example(event: identity_fn.AuthEvent) -> None:
         self.assertIsNone(captured_event.data.password_salt)
         self.assertIsNone(captured_event.data.tokens_valid_after_time)
         self.assertIsNone(captured_event.data.multi_factor)
+
+    def test_on_user_created_is_not_tenant(self) -> None:
+        called_uids: list[str] = []
+        init_calls = 0
+
+        @core.init
+        def init_fn() -> None:
+            nonlocal init_calls
+            init_calls += 1
+
+        @identity_fn.on_user_created(tenant_id=identity_fn.IS_NOT_TENANT)
+        def example(event: identity_fn.AuthEvent) -> None:
+            called_uids.append(event.data.uid)
+
+        endpoint = getattr(example, "__firebase_endpoint__", None)
+        assert endpoint is not None
+        self.assertEqual(endpoint.eventTrigger["eventFilters"], {})
+
+        rendered_yaml = yaml.safe_load(serving.functions_as_yaml({"example": example}))
+        self.assertEqual(
+            rendered_yaml["endpoints"]["example"]["eventTrigger"]["eventFilters"],
+            {},
+        )
+
+        # Should ignore events with top-level tenantid CloudEvent attribute
+        # before unpacking the payload (and not run @core.init)
+        tenant_attr_ce = CloudEvent(
+            attributes={
+                "specversion": "1.0",
+                "id": "event-tenant-attr",
+                "source": "//identitytoolkit.googleapis.com/projects/my-project",
+                "type": "google.firebase.auth.user.v2.created",
+                "time": "2023-01-01T00:00:00Z",
+                "tenantid": "some-tenant",
+            },
+            data=None,
+        )
+        example(tenant_attr_ce)  # type: ignore[arg-type]
+        self.assertEqual(called_uids, [])
+        self.assertEqual(init_calls, 0)
+
+        # Should execute for non-tenant events (and run @core.init)
+        non_tenant_ce = CloudEvent(
+            attributes={
+                "specversion": "1.0",
+                "id": "event-no-tenant",
+                "source": "//identitytoolkit.googleapis.com/projects/my-project",
+                "type": "google.firebase.auth.user.v2.created",
+                "time": "2023-01-01T00:00:00Z",
+            },
+            data={"value": {"uid": "user-no-tenant"}},
+        )
+        example(non_tenant_ce)  # type: ignore[arg-type]
+        self.assertEqual(called_uids, ["user-no-tenant"])
+        self.assertEqual(init_calls, 1)
+
+        # Also verify on_user_deleted with IS_NOT_TENANT
+        deleted_uids: list[str] = []
+
+        @identity_fn.on_user_deleted(tenant_id=identity_fn.IS_NOT_TENANT)
+        def example_deleted(event: identity_fn.AuthEvent) -> None:
+            deleted_uids.append(event.data.uid)
+
+        delete_endpoint = getattr(example_deleted, "__firebase_endpoint__", None)
+        assert delete_endpoint is not None
+        self.assertEqual(delete_endpoint.eventTrigger["eventFilters"], {})
+
+        non_tenant_deleted_ce = CloudEvent(
+            attributes={
+                "specversion": "1.0",
+                "id": "event-del-no-tenant",
+                "source": "//identitytoolkit.googleapis.com/projects/my-project",
+                "type": "google.firebase.auth.user.v2.deleted",
+                "time": "2023-01-01T00:00:00Z",
+            },
+            data={"value": None, "oldValue": {"uid": "deleted-no-tenant"}},
+        )
+        tenant_deleted_ce = CloudEvent(
+            attributes={
+                "specversion": "1.0",
+                "id": "event-del-tenant",
+                "source": "//identitytoolkit.googleapis.com/projects/my-project",
+                "type": "google.firebase.auth.user.v2.deleted",
+                "time": "2023-01-01T00:00:00Z",
+                "tenantid": "some-tenant",
+            },
+            data={"value": None, "oldValue": {"uid": "deleted-with-tenant"}},
+        )
+        example_deleted(non_tenant_deleted_ce)  # type: ignore[arg-type]
+        example_deleted(tenant_deleted_ce)  # type: ignore[arg-type]
+        self.assertEqual(deleted_uids, ["deleted-no-tenant"])
+
+        # Rejects other Sentinel instances such as RESET_VALUE
+        with self.assertRaises(ValueError):
+
+            @identity_fn.on_user_created(tenant_id=options.RESET_VALUE)  # type: ignore[arg-type]
+            def _invalid_sentinel(_event: identity_fn.AuthEvent) -> None:
+                pass