diff --git a/stdlib/@tests/stubtest_allowlists/common.txt b/stdlib/@tests/stubtest_allowlists/common.txt
index cba2f2149c02..6fc81bc8e4a2 100644
--- a/stdlib/@tests/stubtest_allowlists/common.txt
+++ b/stdlib/@tests/stubtest_allowlists/common.txt
@@ -500,7 +500,6 @@ types.MethodType.__closure__  # read-only but not actually a property; stubtest
 types.MethodType.__defaults__  # read-only but not actually a property; stubtest thinks it doesn't exist.
 types.ModuleType.__dict__  # read-only but not actually a property; stubtest thinks it's a mutable attribute.
 types.ModuleType.__getattr__  # this doesn't exist at runtime
-types.SimpleNamespace.__init__  # class doesn't accept positional arguments but has default C signature
 
 # sys attributes that are not always defined
 sys.gettotalrefcount  # Available on python debug builds
diff --git a/stdlib/@tests/stubtest_allowlists/py310.txt b/stdlib/@tests/stubtest_allowlists/py310.txt
index 25e238c547c4..8e77280791fa 100644
--- a/stdlib/@tests/stubtest_allowlists/py310.txt
+++ b/stdlib/@tests/stubtest_allowlists/py310.txt
@@ -249,6 +249,7 @@ pkgutil.ImpImporter\..*
 pkgutil.ImpLoader\..*
 
 types.CodeType.replace  # stubtest thinks default values are None but None doesn't work at runtime
+types.SimpleNamespace.__init__ # class doesn't accept positional arguments but has default C signature
 
 # These enums derive from (str, Enum)
 pstats.SortKey.__new__
diff --git a/stdlib/@tests/stubtest_allowlists/py311.txt b/stdlib/@tests/stubtest_allowlists/py311.txt
index 25a12ada6c2b..aac0d4820b62 100644
--- a/stdlib/@tests/stubtest_allowlists/py311.txt
+++ b/stdlib/@tests/stubtest_allowlists/py311.txt
@@ -135,6 +135,7 @@ inspect._ParameterKind.description  # Still exists, but stubtest can't see it
 os.PathLike.__class_getitem__  # PathLike is a protocol; we don't expect all PathLike classes to implement class_getitem
 poplib.POP3_SSL.stls  # bad declaration of inherited function. See poplib.pyi
 types.GenericAlias.__call__  # Would be complicated to fix properly, Any could silence problems. #6392
+types.SimpleNamespace.__init__ # class doesn't accept positional arguments but has default C signature
 types.GenericAlias.__getattr__
 types.GenericAlias.__mro_entries__
 weakref.ProxyType.__reversed__  # Doesn't really exist
diff --git a/stdlib/@tests/stubtest_allowlists/py312.txt b/stdlib/@tests/stubtest_allowlists/py312.txt
index 469efb38be4a..616fe8cf620e 100644
--- a/stdlib/@tests/stubtest_allowlists/py312.txt
+++ b/stdlib/@tests/stubtest_allowlists/py312.txt
@@ -184,6 +184,9 @@ typing.ParamSpecKwargs.__mro_entries__
 typing.TypeVar.__mro_entries__
 typing.TypeVarTuple.__mro_entries__
 
+# class doesn't accept positional arguments but has default C signature
+types.SimpleNamespace.__init__
+
 # TODO: mypy should infer that this attribute is inherited from builtins.type;
 # why doesn't it infer this?
 typing.SupportsAbs.__type_params__
diff --git a/stdlib/@tests/stubtest_allowlists/py38.txt b/stdlib/@tests/stubtest_allowlists/py38.txt
index cb1b23328a35..893efaae4a38 100644
--- a/stdlib/@tests/stubtest_allowlists/py38.txt
+++ b/stdlib/@tests/stubtest_allowlists/py38.txt
@@ -206,6 +206,7 @@ types.GetSetDescriptorType.__get__
 types.MemberDescriptorType.__get__
 types.MethodDescriptorType.__get__
 types.WrapperDescriptorType.__get__
+types.SimpleNamespace.__init__  # class doesn't accept positional arguments but has default C signature
 multiprocessing.managers.DictProxy.clear
 multiprocessing.managers.DictProxy.popitem
 
diff --git a/stdlib/@tests/stubtest_allowlists/py39.txt b/stdlib/@tests/stubtest_allowlists/py39.txt
index ceac9d170702..5203888299e9 100644
--- a/stdlib/@tests/stubtest_allowlists/py39.txt
+++ b/stdlib/@tests/stubtest_allowlists/py39.txt
@@ -223,6 +223,7 @@ pkgutil.ImpImporter\..*
 pkgutil.ImpLoader\..*
 
 types.CodeType.replace  # stubtest thinks default values are None but None doesn't work at runtime
+types.SimpleNamespace.__init__  # class doesn't accept positional arguments but has default C signature
 
 # These enums derive from (str, Enum)
 pstats.SortKey.__new__
diff --git a/stdlib/@tests/test_cases/check_types.py b/stdlib/@tests/test_cases/check_types.py
new file mode 100644
index 000000000000..3654ca86447a
--- /dev/null
+++ b/stdlib/@tests/test_cases/check_types.py
@@ -0,0 +1,27 @@
+import sys
+import types
+from collections import UserDict
+
+# test `types.SimpleNamespace`
+
+# Valid:
+types.SimpleNamespace()
+types.SimpleNamespace(x=1, y=2)
+
+if sys.version_info >= (3, 13):
+    types.SimpleNamespace(())
+    types.SimpleNamespace([])
+    types.SimpleNamespace([("x", "y"), ("z", 1)])
+    types.SimpleNamespace({})
+    types.SimpleNamespace(UserDict({"x": 1, "y": 2}))
+
+
+# Invalid:
+types.SimpleNamespace(1)  # type: ignore
+types.SimpleNamespace([1])  # type: ignore
+types.SimpleNamespace([["x"]])  # type: ignore
+types.SimpleNamespace(**{1: 2})  # type: ignore
+types.SimpleNamespace({1: 2})  # type: ignore
+types.SimpleNamespace([[1, 2]])  # type: ignore
+types.SimpleNamespace(UserDict({1: 2}))  # type: ignore
+types.SimpleNamespace([[[], 2]])  # type: ignore
diff --git a/stdlib/types.pyi b/stdlib/types.pyi
index df2d853a630c..a569b55efa23 100644
--- a/stdlib/types.pyi
+++ b/stdlib/types.pyi
@@ -312,7 +312,11 @@ class MappingProxyType(Mapping[_KT, _VT_co]):
 
 class SimpleNamespace:
     __hash__: ClassVar[None]  # type: ignore[assignment]
-    def __init__(self, **kwargs: Any) -> None: ...
+    if sys.version_info >= (3, 13):
+        def __init__(self, mapping_or_iterable: Mapping[str, Any] | Iterable[tuple[str, Any]] = (), /, **kwargs: Any) -> None: ...
+    else:
+        def __init__(self, **kwargs: Any) -> None: ...
+
     def __eq__(self, value: object, /) -> bool: ...
     def __getattribute__(self, name: str, /) -> Any: ...
     def __setattr__(self, name: str, value: Any, /) -> None: ...