diff --git a/src/flake8/options/manager.py b/src/flake8/options/manager.py
index 13c7969..d076862 100644
--- a/src/flake8/options/manager.py
+++ b/src/flake8/options/manager.py
@@ -28,6 +28,8 @@ _optparse_callable_map = {
     "float": float,
     "complex": complex,
     "choice": _ARG.NO,
+    # optparse allows this but does not document it
+    "str": str,
 }  # type: Dict[str, Union[Type[Any], _ARG]]
 
 
diff --git a/tests/unit/test_option_manager.py b/tests/unit/test_option_manager.py
index b97a9a6..f2ee4f9 100644
--- a/tests/unit/test_option_manager.py
+++ b/tests/unit/test_option_manager.py
@@ -303,6 +303,8 @@ def test_optparse_normalize_callback_option_legacy(optmanager):
         ('string', 'foo', 'foo'),
         ('float', '1.5', 1.5),
         ('complex', '1+5j', 1 + 5j),
+        # optparse allows this but does not document it
+        ('str', 'foo', 'foo'),
     ),
 )
 def test_optparse_normalize_types(optmanager, type_s, input_val, expected):