diff --git a/libraries/AP_HAL_SITL/UARTDriver.cpp b/libraries/AP_HAL_SITL/UARTDriver.cpp
index 401fb17a0c..b30dad3e79 100644
--- a/libraries/AP_HAL_SITL/UARTDriver.cpp
+++ b/libraries/AP_HAL_SITL/UARTDriver.cpp
@@ -787,6 +787,7 @@ uint16_t UARTDriver::read_from_async_csv(uint8_t *buffer, uint16_t space)
 
 void UARTDriver::handle_writing_from_writebuffer_to_device()
 {
+    WITH_SEMAPHORE(write_mtx);
     if (!_connected) {
         _check_reconnect();
         return;
diff --git a/libraries/AP_HAL_SITL/UARTDriver.h b/libraries/AP_HAL_SITL/UARTDriver.h
index cac1dc4e7a..45c00de18b 100644
--- a/libraries/AP_HAL_SITL/UARTDriver.h
+++ b/libraries/AP_HAL_SITL/UARTDriver.h
@@ -110,6 +110,8 @@ private:
     uint32_t last_read_tick_us;
     uint32_t last_write_tick_us;
 
+    HAL_Semaphore write_mtx;
+
     SITL::SerialDevice *_sim_serial_device;
 
     struct {