diff --git a/django/contrib/postgres/forms/jsonb.py b/django/contrib/postgres/forms/jsonb.py
index 83a62a8b7c..719de3ae66 100644
--- a/django/contrib/postgres/forms/jsonb.py
+++ b/django/contrib/postgres/forms/jsonb.py
@@ -11,6 +11,10 @@ class InvalidJSONInput(six.text_type):
     pass
 
 
+class JSONString(six.text_type):
+    pass
+
+
 class JSONField(forms.CharField):
     default_error_messages = {
         'invalid': _("'%(value)s' value must be valid JSON."),
@@ -22,14 +26,20 @@ class JSONField(forms.CharField):
             return value
         if value in self.empty_values:
             return None
+        elif isinstance(value, (list, dict, int, float, JSONString)):
+            return value
         try:
-            return json.loads(value)
+            converted = json.loads(value)
         except ValueError:
             raise forms.ValidationError(
                 self.error_messages['invalid'],
                 code='invalid',
                 params={'value': value},
             )
+        if isinstance(converted, six.text_type):
+            return JSONString(converted)
+        else:
+            return converted
 
     def bound_data(self, data, initial):
         if self.disabled:
diff --git a/tests/postgres_tests/test_json.py b/tests/postgres_tests/test_json.py
index 78dded31a9..9691329a49 100644
--- a/tests/postgres_tests/test_json.py
+++ b/tests/postgres_tests/test_json.py
@@ -365,3 +365,13 @@ class TestFormField(PostgreSQLTestCase):
 
         field = CustomJSONField()
         self.assertIsInstance(field.widget, widgets.Input)
+
+    def test_already_converted_value(self):
+        field = forms.JSONField(required=False)
+        tests = [
+            '["a", "b", "c"]', '{"a": 1, "b": 2}', '1', '1.5', '"foo"',
+            'true', 'false', 'null',
+        ]
+        for json_string in tests:
+            val = field.clean(json_string)
+            self.assertEqual(field.clean(val), val)