diff --git a/pydantic/main.py b/pydantic/main.py
index b6f0928bb20..fb57dc36a22 100644
--- a/pydantic/main.py
+++ b/pydantic/main.py
@@ -136,8 +136,6 @@ def is_valid_field(name: str) -> bool:
 def validate_custom_root_type(fields: Dict[str, ModelField]) -> None:
     if len(fields) > 1:
         raise ValueError('__root__ cannot be mixed with other fields')
-    if fields[ROOT_KEY].shape == SHAPE_MAPPING:
-        raise TypeError('custom root type cannot allow mapping')
 
 
 UNTOUCHED_TYPES = FunctionType, property, type, classmethod, staticmethod
@@ -246,7 +244,24 @@ def __new__(mcs, name, bases, namespace, **kwargs):  # noqa C901
             '__repr__': Representation.__repr__,
             **{n: v for n, v in namespace.items() if n not in fields},
         }
-        return super().__new__(mcs, name, bases, new_namespace, **kwargs)
+        cls = super().__new__(mcs, name, bases, new_namespace, **kwargs)
+        if _custom_root_type:
+            _setup_custom_root_type(cls)
+        return cls
+
+
+def _setup_custom_root_type(cls: Type['Model']) -> None:
+    original_parse_obj = cls.parse_obj
+
+    root_field = cls.__fields__[ROOT_KEY]
+    root_field_is_mapping = root_field.shape == SHAPE_MAPPING
+
+    def parse_obj(obj: Any) -> 'Model':
+        if not isinstance(obj, dict) or root_field_is_mapping:
+            obj = {ROOT_KEY: obj}
+        return original_parse_obj(obj)
+
+    setattr(cls, 'parse_obj', staticmethod(parse_obj))
 
 
 class BaseModel(metaclass=ModelMetaclass):
@@ -379,14 +394,11 @@ def json(
     @classmethod
     def parse_obj(cls: Type['Model'], obj: Any) -> 'Model':
         if not isinstance(obj, dict):
-            if cls.__custom_root_type__:
-                obj = {ROOT_KEY: obj}
-            else:
-                try:
-                    obj = dict(obj)
-                except (TypeError, ValueError) as e:
-                    exc = TypeError(f'{cls.__name__} expected dict not {type(obj).__name__}')
-                    raise ValidationError([ErrorWrapper(exc, loc=ROOT_KEY)], cls) from e
+            try:
+                obj = dict(obj)
+            except (TypeError, ValueError) as e:
+                exc = TypeError(f'{cls.__name__} expected dict not {type(obj).__name__}')
+                raise ValidationError([ErrorWrapper(exc, loc=ROOT_KEY)], cls) from e
         return cls(**obj)
 
     @classmethod
diff --git a/tests/test_main.py b/tests/test_main.py
index 29c7f948c4f..9b0a620af85 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -900,10 +900,10 @@ class MyModel(BaseModel):
 
 
 def test_parse_root_as_mapping():
-    with pytest.raises(TypeError, match='custom root type cannot allow mapping'):
+    class MyModel(BaseModel):
+        __root__: Mapping[str, str]
 
-        class MyModel(BaseModel):
-            __root__: Mapping[str, str]
+    assert MyModel.parse_obj({1: 2}).__root__ == {'1': '2'}
 
 
 def test_untouched_types():