diff --git a/stdlib/2/itertools.pyi b/stdlib/2/itertools.pyi
index a37468d81159..e7a44f293729 100644
--- a/stdlib/2/itertools.pyi
+++ b/stdlib/2/itertools.pyi
@@ -35,7 +35,7 @@ def ifilterfalse(predicate: Optional[Callable[[_T], Any]],
 def groupby(iterable: Iterable[_T]) -> Iterator[Tuple[_T, Iterator[_T]]]: ...
 @overload
 def groupby(iterable: Iterable[_T],
-            key: Callable[[_T], _S]) -> Iterator[Tuple[_S, Iterator[_T]]]: ...
+            key: Optional[Callable[[_T], _S]]) -> Iterator[Tuple[_S, Iterator[_T]]]: ...
 
 @overload
 def islice(iterable: Iterable[_T], stop: Optional[int]) -> Iterator[_T]: ...
diff --git a/stdlib/3/itertools.pyi b/stdlib/3/itertools.pyi
index 2b56406e824e..c990f4a06323 100644
--- a/stdlib/3/itertools.pyi
+++ b/stdlib/3/itertools.pyi
@@ -38,7 +38,7 @@ def filterfalse(predicate: Optional[Predicate[_T]],
 def groupby(iterable: Iterable[_T]) -> Iterator[Tuple[_T, Iterator[_T]]]: ...
 @overload
 def groupby(iterable: Iterable[_T],
-            key: Callable[[_T], _S]) -> Iterator[Tuple[_S, Iterator[_T]]]: ...
+            key: Optional[Callable[[_T], _S]]) -> Iterator[Tuple[_S, Iterator[_T]]]: ...
 
 @overload
 def islice(iterable: Iterable[_T], stop: Optional[int]) -> Iterator[_T]: ...