diff --git a/src/attr/__init__.pyi b/src/attr/__init__.pyi
index 610eced79..a2b23dcc6 100644
--- a/src/attr/__init__.pyi
+++ b/src/attr/__init__.pyi
@@ -51,6 +51,7 @@ _OnSetAttrArgType = Union[
 _FieldTransformer = Callable[
     [type, List[Attribute[Any]]], List[Attribute[Any]]
 ]
+_CompareWithType = Callable[[Any, Any], bool]
 # FIXME: in reality, if multiple validators are passed they must be in a list
 # or tuple, but those are invariant and so would prevent subtypes of
 # _ValidatorType from working when passed in a list or tuple.