diff --git a/ports/stm32/Makefile b/ports/stm32/Makefile
index e99d386e49..141aadcf38 100644
--- a/ports/stm32/Makefile
+++ b/ports/stm32/Makefile
@@ -474,7 +474,8 @@ USBDEV_SRC_C += $(addprefix $(USBDEV_DIR)/,\
 	)
 
 ifeq ($(MICROPY_SSL_MBEDTLS),1)
-CFLAGS += -DMBEDTLS_CONFIG_FILE='"mbedtls/mbedtls_config.h"'
+MBEDTLS_CONFIG_FILE ?= '"mbedtls/mbedtls_config.h"'
+CFLAGS += -DMBEDTLS_CONFIG_FILE=$(MBEDTLS_CONFIG_FILE)
 LIB_SRC_C += mbedtls/mbedtls_port.c
 endif