diff --git a/libraries/AP_HAL_SITL/Scheduler.h b/libraries/AP_HAL_SITL/Scheduler.h
index 829a1dca5e..35484421c3 100644
--- a/libraries/AP_HAL_SITL/Scheduler.h
+++ b/libraries/AP_HAL_SITL/Scheduler.h
@@ -4,6 +4,7 @@
 #if CONFIG_HAL_BOARD == HAL_BOARD_SITL
 #include "AP_HAL_SITL_Namespace.h"
 #include <sys/time.h>
+#include <pthread.h>
 
 #define SITL_SCHEDULER_MAX_TIMER_PROCS 4