diff --git a/thread.c b/thread.c
index 74cdf4f3c7..4578cbe7f6 100644
--- a/thread.c
+++ b/thread.c
@@ -77,6 +77,9 @@
 #define RUBY_THREAD_PRIORITY_MIN -3
 #endif
 
+#if defined HAVE_VALGRIND_MEMCHECK_H && __APPLE__
+#define THREAD_DEBUG -1
+#endif
 #ifndef THREAD_DEBUG
 #define THREAD_DEBUG 0
 #endif