diff --git a/django/contrib/admin/options.py b/django/contrib/admin/options.py
index 0d10d29a04..eb2c8cd094 100644
--- a/django/contrib/admin/options.py
+++ b/django/contrib/admin/options.py
@@ -2126,7 +2126,8 @@ class InlineModelAdmin(BaseModelAdmin):
             queryset = queryset.none()
         return queryset
 
-    def has_add_permission(self, request, obj):
+    def has_add_permission(self, request, obj=None):
+        # RemovedInDjango31Warning: obj becomes a mandatory argument.
         if self.opts.auto_created:
             # We're checking the rights to an auto-created intermediate model,
             # which doesn't have its own individual permissions. The user needs
diff --git a/docs/ref/contrib/admin/index.txt b/docs/ref/contrib/admin/index.txt
index 7a14d694a6..c163dc9f2e 100644
--- a/docs/ref/contrib/admin/index.txt
+++ b/docs/ref/contrib/admin/index.txt
@@ -2413,7 +2413,9 @@ The ``InlineModelAdmin`` class adds or customizes:
 
     .. versionchanged:: 2.1
 
-        The ``obj`` argument was added.
+        The ``obj`` argument was added. During the deprecation period, it may
+        also be ``None`` if third-party calls to ``has_add_permission()`` don't
+        provide it.
 
 .. method:: InlineModelAdmin.has_change_permission(request, obj=None)
 
diff --git a/docs/releases/2.1.6.txt b/docs/releases/2.1.6.txt
index 894eb160b2..d5f342f546 100644
--- a/docs/releases/2.1.6.txt
+++ b/docs/releases/2.1.6.txt
@@ -9,4 +9,6 @@ Django 2.1.6 several bugs in 2.1.5.
 Bugfixes
 ========
 
-* ...
+* Made the ``obj`` argument of ``InlineModelAdmin.has_add_permission()``
+  optional to restore backwards compatibility with third-party code that
+  doesn't provide it (:ticket:`30097`).
diff --git a/tests/modeladmin/tests.py b/tests/modeladmin/tests.py
index 6934bf2b3c..0d78dd9c21 100644
--- a/tests/modeladmin/tests.py
+++ b/tests/modeladmin/tests.py
@@ -734,6 +734,10 @@ class ModelAdminPermissionTests(SimpleTestCase):
         def has_perm(self, perm):
             return perm == 'modeladmin.add_band'
 
+    class MockAddUserWithInline(MockUser):
+        def has_perm(self, perm):
+            return perm == 'modeladmin.add_concert'
+
     class MockChangeUser(MockUser):
         def has_perm(self, perm):
             return perm == 'modeladmin.change_band'
@@ -793,6 +797,26 @@ class ModelAdminPermissionTests(SimpleTestCase):
         self.assertEqual(len(inline_instances), 1)
         self.assertIsInstance(inline_instances[0], ConcertInline)
 
+    def test_inline_has_add_permission_without_obj(self):
+        # This test will be removed in Django 3.1 when `obj` becomes a required
+        # argument of has_add_permission() (#27991).
+        class ConcertInline(TabularInline):
+            model = Concert
+
+            def has_add_permission(self, request):
+                return super().has_add_permission(request)
+
+        class BandAdmin(ModelAdmin):
+            inlines = [ConcertInline]
+
+        ma = BandAdmin(Band, AdminSite())
+        request = MockRequest()
+        request.user = self.MockAddUserWithInline()
+        band = Band(name='The Doors', bio='', sign_date=date(1965, 1, 1))
+        inline_instances = ma.get_inline_instances(request, band)
+        self.assertEqual(len(inline_instances), 1)
+        self.assertIsInstance(inline_instances[0], ConcertInline)
+
     def test_has_change_permission(self):
         """
         has_change_permission returns True for users who can edit objects and