diff --git a/lib/libunbound/Makefile b/lib/libunbound/Makefile
index 4ce8fcb4ab9..0b7f92e3318 100644
--- a/lib/libunbound/Makefile
+++ b/lib/libunbound/Makefile
@@ -14,6 +14,7 @@ PACKAGE=	unbound
 
 CFLAGS+= -I${UNBOUNDDIR} -I${LDNSDIR} -I${.OBJDIR}
 CFLAGS+= -I${SRCTOP}/usr.sbin/unbound
+CFLAGS+= -DOPENSSL_API_COMPAT=0x10100000L
 
 SRCS=	alloc.c as112.c authzone.c autotrust.c cachedb.c config_file.c \
 	configlexer.l configparser.y context.c dname.c dns.c dns64.c \