diff --git a/gc.c b/gc.c
index c19b77b33e..0b5f792839 100644
--- a/gc.c
+++ b/gc.c
@@ -5412,8 +5412,6 @@ check_stack_for_moved(rb_objspace_t *objspace)
     each_machine_stack_value(ec, revert_machine_stack_references);
 }
 
-static void gc_mode_transition(rb_objspace_t *objspace, enum gc_mode mode);
-
 static void
 gc_compact_finish(rb_objspace_t *objspace)
 {