diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-05-02-08-36-03.bpo-33391.z4a7rb.rst b/Misc/NEWS.d/next/Core and Builtins/2018-05-02-08-36-03.bpo-33391.z4a7rb.rst
new file mode 100644
index 00000000000..ab17aa408c0
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2018-05-02-08-36-03.bpo-33391.z4a7rb.rst	
@@ -0,0 +1 @@
+Fix a leak in set_symmetric_difference().
diff --git a/Objects/setobject.c b/Objects/setobject.c
index 80101dda9be..82b58382081 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -1744,8 +1744,10 @@ set_symmetric_difference(PySetObject *so, PyObject *other)
     if (otherset == NULL)
         return NULL;
     rv = set_symmetric_difference_update(otherset, (PyObject *)so);
-    if (rv == NULL)
+    if (rv == NULL) {
+        Py_DECREF(otherset);
         return NULL;
+    }
     Py_DECREF(rv);
     return (PyObject *)otherset;
 }