diff --git a/stdlib/3/itertools.pyi b/stdlib/3/itertools.pyi
index 8296d6282..889d583c5 100644
--- a/stdlib/3/itertools.pyi
+++ b/stdlib/3/itertools.pyi
@@ -17,7 +17,7 @@ def repeat(object: _T) -> Iterator[_T]: ...
 @overload
 def repeat(object: _T, times: int) -> Iterator[_T]: ...
 
-def accumulate(iterable: Iterable[_T]) -> Iterator[_T]: ...
+def accumulate(iterable: Iterable[_T], func: Callable[[_T, _T], _T] = ...) -> Iterator[_T]: ...
 
 class chain(Iterator[_T], Generic[_T]):
     def __init__(self, *iterables: Iterable[_T]) -> None: ...