diff --git a/drf_spectacular/openapi.py b/drf_spectacular/openapi.py
index ed00a828..534a5299 100644
--- a/drf_spectacular/openapi.py
+++ b/drf_spectacular/openapi.py
@@ -699,7 +699,10 @@ def _map_serializer_field(self, field, direction, bypass_extensions=False):
             # read_only fields do not have a Manager by design. go around and get field
             # from parent. also avoid calling Manager. __bool__ as it might be customized
             # to hit the database.
-            if getattr(field, 'queryset', None) is not None:
+            if not is_slug and getattr(field, 'pk_field') is not None:
+                schema = self._map_serializer_field(field.pk_field, direction)
+                return append_meta(schema, meta)
+            elif getattr(field, 'queryset', None) is not None:
                 if is_slug:
                     model = field.queryset.model
                     source = [field.slug_field]
@@ -718,7 +721,7 @@ def _map_serializer_field(self, field, direction, bypass_extensions=False):
                         f'Could not derive type for under-specified {field.__class__.__name__} '
                         f'"{field.field_name}". The serializer has no associated model (Meta class) '
                         f'and this particular field has no type without a model association. Consider '
-                        f'changing the field or adding a Meta class. defaulting to string.'
+                        f'changing the field or adding a Meta class. Defaulting to string.'
                     )
                     return append_meta(build_basic_type(OpenApiTypes.STR), meta)
 
diff --git a/tests/test_regressions.py b/tests/test_regressions.py
index 0dd53299..eb7f3457 100644
--- a/tests/test_regressions.py
+++ b/tests/test_regressions.py
@@ -3444,3 +3444,20 @@ class XViewset(viewsets.ModelViewSet):
             'readOnly': True
         }
     }
+
+
+def test_primary_key_related_field_with_custom_pk_field(no_warnings):
+    class XSerializer(serializers.Serializer):
+        field = serializers.PrimaryKeyRelatedField(
+            read_only=True,
+            pk_field=serializers.IntegerField(),
+        )
+
+    class XViewset(viewsets.ModelViewSet):
+        serializer_class = XSerializer
+        queryset = SimpleModel.objects.all()
+
+    schema = generate_schema('/x', XViewset)
+    assert schema['components']['schemas']['X']['properties']['field'] == {
+        'readOnly': True, 'type': 'integer'
+    }