diff --git a/package/proftpd/proftpd.mk b/package/proftpd/proftpd.mk
index 84a499a5c0..d450489cb0 100644
--- a/package/proftpd/proftpd.mk
+++ b/package/proftpd/proftpd.mk
@@ -27,6 +27,14 @@ PROFTPD_CONF_OPTS = \
 	--with-gnu-ld \
 	--without-openssl-cmdline
 
+# source code contains a number of variables named 'bool', which
+# conflicts with the C23 keyword. Fixed upstream in 1.3.9 with
+# https://github.com/proftpd/proftpd/commit/61be7eb14f200b97804a3cfa85fed51661067c62
+# so can be dropped when bumping to that
+ifeq ($(BR2_TOOLCHAIN_GCC_AT_LEAST_15),y)
+PROFTPD_CONF_ENV += CFLAGS="$(TARGET_CFLAGS) -std=gnu18"
+endif
+
 ifeq ($(BR2_PACKAGE_LIBIDN2),y)
 PROFTPD_DEPENDENCIES += libidn2
 endif