diff --git a/ChangeLog.d/arc4random_buf-implicit.txt b/ChangeLog.d/arc4random_buf-implicit.txt
new file mode 100644
index 0000000..81c245e
--- /dev/null
+++ b/ChangeLog.d/arc4random_buf-implicit.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Make arc4random_buf available on NetBSD and OpenBSD when _POSIX_C_SOURCE is
+     defined. Fix contributed in #3571.
diff --git a/tests/src/random.c b/tests/src/random.c
index 3345f78..78419c4 100644
--- a/tests/src/random.c
+++ b/tests/src/random.c
@@ -24,6 +24,15 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/*
+ * for arc4random_buf() from <stdlib.h>
+ */
+#if defined(__NetBSD__)
+#define _NETBSD_SOURCE 1
+#elif defined(__OpenBSD__)
+#define _BSD_SOURCE 1
+#endif
+
 #include <test/macros.h>
 #include <test/random.h>
 #include <string.h>