diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index e5e11f472a1..b55de3d7665 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -1791,13 +1791,18 @@ class Query(object):
         existing immediate values, but respects existing deferrals.)
         """
         existing, defer = self.deferred_loading
+        field_names = set(field_names)
+        if 'pk' in field_names:
+            field_names.remove('pk')
+            field_names.add(self.model._meta.pk.name)
+
         if defer:
             # Remove any existing deferred names from the current set before
             # setting the new names.
-            self.deferred_loading = set(field_names).difference(existing), False
+            self.deferred_loading = field_names.difference(existing), False
         else:
             # Replace any existing "immediate load" field names.
-            self.deferred_loading = set(field_names), False
+            self.deferred_loading = field_names, False
 
     def get_loaded_field_names(self):
         """
diff --git a/tests/modeltests/defer/tests.py b/tests/modeltests/defer/tests.py
index 5f6c53dee2e..ccf19fd5ae7 100644
--- a/tests/modeltests/defer/tests.py
+++ b/tests/modeltests/defer/tests.py
@@ -28,11 +28,18 @@ class DeferTests(TestCase):
         self.assert_delayed(qs.only("name")[0], 2)
         self.assert_delayed(qs.defer("related__first")[0], 0)
 
+        # Using 'pk' with only() should result in 3 deferred fields, namely all
+        # of them except the model's primary key see #15494
+        self.assert_delayed(qs.only("pk")[0], 3)
+
         obj = qs.select_related().only("related__first")[0]
         self.assert_delayed(obj, 2)
 
         self.assertEqual(obj.related_id, s1.pk)
 
+        # You can use 'pk' with reverse foreign key lookups.
+        self.assert_delayed(s1.primary_set.all().only('pk')[0], 3)
+
         self.assert_delayed(qs.defer("name").extra(select={"a": 1})[0], 1)
         self.assert_delayed(qs.extra(select={"a": 1}).defer("name")[0], 1)
         self.assert_delayed(qs.defer("name").defer("value")[0], 2)
@@ -135,3 +142,4 @@ class DeferTests(TestCase):
         self.assertEqual(obj.other, "bar")
         obj.name = "bb"
         obj.save()
+