diff --git a/lib/libthr/Makefile b/lib/libthr/Makefile
index 062f5eec5d8..6337c87d5d6 100644
--- a/lib/libthr/Makefile
+++ b/lib/libthr/Makefile
@@ -51,8 +51,10 @@ SYMBOL_MAPS=${.CURDIR}/pthread.map
 
 MAN=	libthr.3
 
+.if ${MK_PTHREADS_ASSERTIONS} != "no"
 # enable extra internal consistency checks
 CFLAGS+=-D_PTHREADS_INVARIANTS
+.endif
 
 PRECIOUSLIB=
 
diff --git a/lib/libthr/thread/thr_mutex.c b/lib/libthr/thread/thr_mutex.c
index 8dccdf4dfa8..2cbfd9a459d 100644
--- a/lib/libthr/thread/thr_mutex.c
+++ b/lib/libthr/thread/thr_mutex.c
@@ -120,7 +120,7 @@ __weak_reference(_pthread_mutex_getyieldloops_np, pthread_mutex_getyieldloops_np
 __weak_reference(_pthread_mutex_isowned_np, pthread_mutex_isowned_np);
 
 static void
-mutex_init_link(struct pthread_mutex *m)
+mutex_init_link(struct pthread_mutex *m __unused)
 {
 
 #if defined(_PTHREADS_INVARIANTS)
diff --git a/share/mk/src.opts.mk b/share/mk/src.opts.mk
index 3c17f931961..0e9b5a052ff 100644
--- a/share/mk/src.opts.mk
+++ b/share/mk/src.opts.mk
@@ -159,6 +159,7 @@ __DEFAULT_YES_OPTIONS = \
     PKGBOOTSTRAP \
     PMC \
     PPP \
+    PTHREADS_ASSERTIONS \
     QUOTAS \
     RADIUS_SUPPORT \
     RBOOTD \
diff --git a/tools/build/options/WITHOUT_PTHREADS_ASSERTIONS b/tools/build/options/WITHOUT_PTHREADS_ASSERTIONS
new file mode 100644
index 00000000000..b0961fa43cb
--- /dev/null
+++ b/tools/build/options/WITHOUT_PTHREADS_ASSERTIONS
@@ -0,0 +1 @@
+Disable debugging assertions in pthreads library.