diff --git a/include/mruby/internal.h b/include/mruby/internal.h
index d0873b3cd..e4be2d1ac 100644
--- a/include/mruby/internal.h
+++ b/include/mruby/internal.h
@@ -136,6 +136,7 @@ int mrb_rational_mark(mrb_state *mrb, struct RBasic *rat);
 #ifdef MRB_USE_SET
 size_t mrb_gc_mark_set(mrb_state *mrb, struct RBasic *set);
 void mrb_gc_free_set(mrb_state *mrb, struct RBasic *set);
+size_t mrb_set_memsize(mrb_value);
 #endif
 
 #ifdef MRUBY_PROC_H
diff --git a/mrbgems/mruby-os-memsize/src/memsize.c b/mrbgems/mruby-os-memsize/src/memsize.c
index 61ae10370..5420dbde4 100644
--- a/mrbgems/mruby-os-memsize/src/memsize.c
+++ b/mrbgems/mruby-os-memsize/src/memsize.c
@@ -156,6 +156,11 @@ os_memsize_of_object(mrb_state* mrb, mrb_value obj)
       size += mrb_objspace_page_slot_size();
 #endif
       break;
+#if defined(MRB_USE_SET)
+    case MRB_TT_SET:
+      size += mrb_set_memsize(obj);
+      break;
+#endif
     case MRB_TT_BIGINT:
 #if defined(MRB_USE_BIGINT)
       size += mrb_bint_memsize(obj);
diff --git a/mrbgems/mruby-set/src/set.c b/mrbgems/mruby-set/src/set.c
index 5e08eacc3..52c49cb1e 100644
--- a/mrbgems/mruby-set/src/set.c
+++ b/mrbgems/mruby-set/src/set.c
@@ -80,6 +80,18 @@ mrb_gc_free_set(mrb_state *mrb, struct RBasic *obj)
   }
 }
 
+size_t
+mrb_set_memsize(mrb_value set)
+{
+
+  size_t size = mrb_objspace_page_slot_size();
+  struct RSet *s = mrb_set_ptr(set);
+  if (s->kh) {
+    size += kh_size(s->kh) * sizeof(mrb_value);
+  }
+  return size;
+}
+
 /* Helper function to check if a value is a Set and return a boolean result */
 static mrb_bool
 set_is_set(mrb_state *mrb, mrb_value obj)