diff --git a/lib/libthr/thread/thr_init.c b/lib/libthr/thread/thr_init.c
index 2f62619ce07..4660627ef86 100644
--- a/lib/libthr/thread/thr_init.c
+++ b/lib/libthr/thread/thr_init.c
@@ -439,6 +439,7 @@ init_private(void)
 	size_t len;
 	int mib[2];
 	char *env, *env_bigstack, *env_splitstack;
+	unsigned int npages;
 
 	_thr_umutex_init(&_mutex_static_lock);
 	_thr_umutex_init(&_cond_static_lock);
@@ -477,7 +478,6 @@ init_private(void)
 		sysctlbyname("kern.smp.cpus", &_thr_is_smp, &len, NULL, 0);
 		_thr_is_smp = (_thr_is_smp > 1);
 		_thr_page_size = getpagesize();
-		_thr_guard_default = (2 * 1024 * 1024);
 		_pthread_attr_default.guardsize_attr = _thr_guard_default;
 		_pthread_attr_default.stacksize_attr = _thr_stack_default;
 		env = getenv("LIBPTHREAD_SPINLOOPS");
@@ -490,6 +490,19 @@ init_private(void)
 		if (env)
 			_thr_queuefifo = atoi(env);
 		TAILQ_INIT(&_thr_atfork_list);
+		/*
+		 * Initialize the random-sized stack guard
+		 * conservatively.
+		 */
+		mib[0] = CTL_KERN;
+		mib[1] = KERN_ARND;
+		len = sizeof(npages);
+		npages = 0;
+		sysctl(mib, 2, &npages, &len, NULL, 0);
+		npages %= GUARD_MAX_PAGES;
+		if (npages < GUARD_MIN_PAGES)
+			npages = GUARD_DEFAULT_SIZE;
+		_thr_guard_default = npages * PAGE_SIZE;
 	}
 	init_once = 1;
 }
diff --git a/lib/libthr/thread/thr_private.h b/lib/libthr/thread/thr_private.h
index e61ee99dcb7..78a74a32256 100644
--- a/lib/libthr/thread/thr_private.h
+++ b/lib/libthr/thread/thr_private.h
@@ -101,6 +101,18 @@ TAILQ_HEAD(mutex_queue, pthread_mutex);
 #define THR_ASSERT(cond, msg)
 #endif
 
+#ifndef GUARD_DEFAULT_SIZE
+#define	GUARD_DEFAULT_SIZE	512
+#endif
+
+#ifndef GUARD_MIN_PAGES
+#define	GUARD_MIN_PAGES	256
+#endif
+
+#ifndef GUARD_MAX_PAGES
+#define	GUARD_MAX_PAGES	1024
+#endif
+
 #ifdef PIC
 # define STATIC_LIB_REQUIRE(name)
 #else