diff --git a/openapi_core/unmarshalling/schemas/unmarshallers.py b/openapi_core/unmarshalling/schemas/unmarshallers.py
index 1a000266..c275a39a 100644
--- a/openapi_core/unmarshalling/schemas/unmarshallers.py
+++ b/openapi_core/unmarshalling/schemas/unmarshallers.py
@@ -6,6 +6,7 @@
 
 from openapi_core.extensions.models.factories import ModelFactory
 from openapi_core.schema.schemas.enums import SchemaFormat, SchemaType
+from openapi_core.schema.schemas.models import Schema
 from openapi_core.schema.schemas.types import NoValue
 from openapi_core.schema_validator._types import (
     is_array, is_bool, is_integer,
@@ -194,11 +195,15 @@ def _unmarshal_properties(self, value=NoValue, one_of_schema=None):
         extra_props = set(value_props_names) - set(all_props_names)
 
         properties = {}
-        if self.schema.additional_properties is not True:
+        if isinstance(self.schema.additional_properties, Schema):
             for prop_name in extra_props:
                 prop_value = value[prop_name]
                 properties[prop_name] = self.unmarshallers_factory.create(
                     self.schema.additional_properties)(prop_value)
+        elif self.schema.additional_properties is True:
+            for prop_name in extra_props:
+                prop_value = value[prop_name]
+                properties[prop_name] = prop_value
 
         for prop_name, prop in iteritems(all_props):
             try:
diff --git a/tests/unit/unmarshalling/test_unmarshal.py b/tests/unit/unmarshalling/test_unmarshal.py
index 7c88c957..ae6c88c2 100644
--- a/tests/unit/unmarshalling/test_unmarshal.py
+++ b/tests/unit/unmarshalling/test_unmarshal.py
@@ -416,3 +416,16 @@ def test_schema_any_one_of(self, unmarshaller_factory):
     def test_schema_any(self, unmarshaller_factory):
         schema = Schema()
         assert unmarshaller_factory(schema)('string') == 'string'
+
+    @pytest.mark.parametrize('value', [
+        {'additional': 1},
+        {'foo': 'bar', 'bar': 'foo'},
+        {'additional': {'bar': 1}},
+    ])
+    @pytest.mark.parametrize('additional_properties', [True, Schema()])
+    def test_schema_free_form_object(
+            self, value, additional_properties, unmarshaller_factory):
+        schema = Schema('object', additional_properties=additional_properties)
+
+        result = unmarshaller_factory(schema)(value)
+        assert result == value