diff --git a/ChangeLog.d/arc4random_buf-implicit.txt b/ChangeLog.d/arc4random_buf-implicit.txt
new file mode 100644
index 000000000..d20e4c848
--- /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. Adopted for LTS branch 2.16 in #3602.
diff --git a/tests/suites/main_test.function b/tests/suites/main_test.function
index f516d2caa..9f366907b 100644
--- a/tests/suites/main_test.function
+++ b/tests/suites/main_test.function
@@ -23,6 +23,15 @@
 #endif
 #endif
 
+/*
+ * for arc4random_buf() from <stdlib.h>
+ */
+#if defined(__NetBSD__)
+#define _NETBSD_SOURCE 1
+#elif defined(__OpenBSD__)
+#define _BSD_SOURCE 1
+#endif
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include <mbedtls/config.h>
 #else