diff --git a/django/contrib/contenttypes/generic.py b/django/contrib/contenttypes/generic.py
index e14aa5ecee..c1db206515 100644
--- a/django/contrib/contenttypes/generic.py
+++ b/django/contrib/contenttypes/generic.py
@@ -465,10 +465,10 @@ class GenericInlineModelAdmin(InlineModelAdmin):
     formset = BaseGenericInlineFormSet
 
     def get_formset(self, request, obj=None, **kwargs):
-        if self.declared_fieldsets:
-            fields = flatten_fieldsets(self.declared_fieldsets)
+        if 'fields' in kwargs:
+            fields = kwargs.pop('fields')
         else:
-            fields = None
+            fields = flatten_fieldsets(self.get_fieldsets(request, obj))
         if self.exclude is None:
             exclude = []
         else:
diff --git a/tests/generic_inline_admin/tests.py b/tests/generic_inline_admin/tests.py
index ac2c191183..efa40ca3d9 100644
--- a/tests/generic_inline_admin/tests.py
+++ b/tests/generic_inline_admin/tests.py
@@ -325,3 +325,23 @@ class GenericInlineModelAdminTest(TestCase):
         self.assertEqual(
             list(list(ma.get_formsets(request))[0]().forms[0].fields),
             ['description', 'keywords', 'id', 'DELETE'])
+
+    def test_get_fieldsets(self):
+        # Test that get_fieldsets is called when figuring out form fields.
+        # Refs #18681.
+        class MediaForm(ModelForm):
+            class Meta:
+                model = Media
+                fields = '__all__'
+
+        class MediaInline(GenericTabularInline):
+            form = MediaForm
+            model = Media
+            can_delete = False
+
+            def get_fieldsets(self, request, obj=None):
+                return [(None, {'fields': ['url', 'description']})]
+
+        ma = MediaInline(Media, self.site)
+        form = ma.get_formset(None).form
+        self.assertEqual(form._meta.fields, ['url', 'description'])