diff --git a/gc.c b/gc.c
index 953b2ed70d..0ca08d53e0 100644
--- a/gc.c
+++ b/gc.c
@@ -1965,6 +1965,8 @@ rb_gc_obj_free_vm_weak_references(VALUE obj)
  *
  *  On multi-ractor mode, if the object is not shareable, it raises
  *  RangeError.
+ *
+ *  This method is deprecated and should no longer be used.
  */
 
 static VALUE
@@ -2012,6 +2014,7 @@ id2ref(VALUE objid)
 static VALUE
 os_id2ref(VALUE os, VALUE objid)
 {
+    rb_category_warn(RB_WARN_CATEGORY_DEPRECATED, "ObjectSpace._id2ref is deprecated");
     return id2ref(objid);
 }