diff --git a/src/class/cdc/cdc_device.c b/src/class/cdc/cdc_device.c
index 89326f71..94473d03 100644
--- a/src/class/cdc/cdc_device.c
+++ b/src/class/cdc/cdc_device.c
@@ -416,6 +416,9 @@ bool cdcd_xfer_cb(uint8_t rhport, uint8_t ep_addr, xfer_result_t result, uint32_
   //       Though maybe the baudrate is not really important !!!
   if ( ep_addr == p_cdc->ep_in )
   {
+    // invoke transmit callback to possibly refill tx fifo
+    if ( tud_cdc_tx_complete_cb ) tud_cdc_tx_complete_cb(itf);
+
     if ( 0 == tud_cdc_n_write_flush(itf) )
     {
       // There is no data left, a ZLP should be sent if
diff --git a/src/class/cdc/cdc_device.h b/src/class/cdc/cdc_device.h
index 9d257bbe..f9692e20 100644
--- a/src/class/cdc/cdc_device.h
+++ b/src/class/cdc/cdc_device.h
@@ -133,6 +133,9 @@ TU_ATTR_WEAK void tud_cdc_rx_cb(uint8_t itf);
 // Invoked when received `wanted_char`
 TU_ATTR_WEAK void tud_cdc_rx_wanted_cb(uint8_t itf, char wanted_char);
 
+// Invoked when space becomes available in TX buffer
+TU_ATTR_WEAK void tud_cdc_tx_complete_cb(uint8_t itf);
+
 // Invoked when line state DTR & RTS are changed via SET_CONTROL_LINE_STATE
 TU_ATTR_WEAK void tud_cdc_line_state_cb(uint8_t itf, bool dtr, bool rts);