diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index 02d887b7f8a..a7e0b52e876 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -1334,7 +1334,10 @@ class GenericIPAddressField(Field):
         return value
 
     def formfield(self, **kwargs):
-        defaults = {'form_class': forms.GenericIPAddressField}
+        defaults = {
+            'protocol': self.protocol,
+            'form_class': forms.GenericIPAddressField,
+        }
         defaults.update(kwargs)
         return super(GenericIPAddressField, self).formfield(**defaults)
 
diff --git a/tests/model_fields/tests.py b/tests/model_fields/tests.py
index 6546c49ec77..1ce5eba3e99 100644
--- a/tests/model_fields/tests.py
+++ b/tests/model_fields/tests.py
@@ -468,3 +468,16 @@ class BinaryFieldTests(test.TestCase):
     def test_max_length(self):
         dm = DataModel(short_data=self.binary_data*4)
         self.assertRaises(ValidationError, dm.full_clean)
+
+class GenericIPAddressFieldTests(test.TestCase):
+    def test_genericipaddressfield_formfield_protocol(self):
+        """
+        Test that GenericIPAddressField with a specified protocol does not
+        generate a formfield with no specified protocol. See #20740.
+        """
+        model_field = models.GenericIPAddressField(protocol='IPv4')
+        form_field = model_field.formfield()
+        self.assertRaises(ValidationError, form_field.clean, '::1')
+        model_field = models.GenericIPAddressField(protocol='IPv6')
+        form_field = model_field.formfield()
+        self.assertRaises(ValidationError, form_field.clean, '127.0.0.1')