diff --git a/tools/timeit.c b/tools/timeit.c
--- a/tools/timeit.c
+++ b/tools/timeit.c
@@ -19,6 +19,10 @@
 #include <sys/time.h>
 #include <sys/wait.h>
 
+#if defined(__APPLE__)
+#include <sys/proc_info.h>
+#endif
+
 /* Enumeration for our exit status codes. */
 enum ExitCode {
   /* \brief Indicates a failure monitoring the target. */
@@ -333,7 +337,17 @@
   if (g_target_data_size_limit != ~(rlim_t) 0) {
     set_resource_limit(RLIMIT_DATA, g_target_data_size_limit);
   }
+
   if (g_target_rss_size_limit != ~(rlim_t) 0) {
+#if defined(__APPLE__)
+    // On Apple platforms, RLIMIT_RSS is mapped to RLIMIT_AS and setting
+    // RLIMIT_AS to a value smaller than the current virtual memory size will
+    // fail. To avoid issues, first get the current size of the virtual memory
+    // and add the user requested limit.
+    struct proc_taskinfo task_info;
+    proc_pidinfo(getpid(), PROC_PIDTASKINFO, 0, &task_info, sizeof(task_info));
+    g_target_rss_size_limit += task_info.pti_virtual_size;
+#endif
     set_resource_limit(RLIMIT_RSS, g_target_rss_size_limit);
   }
   if (g_target_file_size_limit != ~(rlim_t) 0) {