diff --git a/CHANGES.rst b/CHANGES.rst
index 945237cd..0d96a61e 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -5,6 +5,9 @@ Version 2.1.2
 
 Unreleased
 
+-   Fix type annotation for ``json.loads``, it accepts str or bytes.
+    :issue:`4519`
+
 
 Version 2.1.1
 -------------
diff --git a/src/flask/json/__init__.py b/src/flask/json/__init__.py
index 43c018e7..edc9793d 100644
--- a/src/flask/json/__init__.py
+++ b/src/flask/json/__init__.py
@@ -153,7 +153,11 @@ def dump(
     _json.dump(obj, fp, **kwargs)
 
 
-def loads(s: str, app: t.Optional["Flask"] = None, **kwargs: t.Any) -> t.Any:
+def loads(
+    s: t.Union[str, bytes],
+    app: t.Optional["Flask"] = None,
+    **kwargs: t.Any,
+) -> t.Any:
     """Deserialize an object from a string of JSON.
 
     Takes the same arguments as the built-in :func:`json.loads`, with