diff --git a/stdlib/builtins.pyi b/stdlib/builtins.pyi
index 47c4b8403..e487c76c6 100644
--- a/stdlib/builtins.pyi
+++ b/stdlib/builtins.pyi
@@ -868,7 +868,7 @@ class set(MutableSet[_T], Generic[_T]):
     def remove(self, __element: _T) -> None: ...
     def symmetric_difference(self, __s: Iterable[_T]) -> Set[_T]: ...
     def symmetric_difference_update(self, __s: Iterable[_T]) -> None: ...
-    def union(self, *s: Iterable[_T]) -> Set[_T]: ...
+    def union(self, *s: Iterable[_S]) -> Set[_T | _S]: ...
     def update(self, *s: Iterable[_T]) -> None: ...
     def __len__(self) -> int: ...
     def __contains__(self, __o: object) -> bool: ...
@@ -899,7 +899,7 @@ class frozenset(AbstractSet[_T_co], Generic[_T_co]):
     def issubset(self, __s: Iterable[object]) -> bool: ...
     def issuperset(self, __s: Iterable[object]) -> bool: ...
     def symmetric_difference(self, __s: Iterable[_T_co]) -> FrozenSet[_T_co]: ...
-    def union(self, *s: Iterable[_T_co]) -> FrozenSet[_T_co]: ...
+    def union(self, *s: Iterable[_S]) -> FrozenSet[_T_co | _S]: ...
     def __len__(self) -> int: ...
     def __contains__(self, __o: object) -> bool: ...
     def __iter__(self) -> Iterator[_T_co]: ...