diff --git a/tests/unit/test_strlcat.c b/tests/unit/test_strlcat.c
index a6cb9ac20..8fd17bd9f 100644
--- a/tests/unit/test_strlcat.c
+++ b/tests/unit/test_strlcat.c
@@ -25,6 +25,12 @@
  *    SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#ifdef OPENTHREAD_CONFIG_FILE
+#include OPENTHREAD_CONFIG_FILE
+#else
+#include <openthread-config.h>
+#endif
+
 #include <stdio.h>
 #include <stdlib.h>
 #include "utils/wrap_string.h"
diff --git a/tests/unit/test_strlcpy.c b/tests/unit/test_strlcpy.c
index 8a7e7c3a7..6a5195260 100644
--- a/tests/unit/test_strlcpy.c
+++ b/tests/unit/test_strlcpy.c
@@ -25,6 +25,12 @@
  *    SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#ifdef OPENTHREAD_CONFIG_FILE
+#include OPENTHREAD_CONFIG_FILE
+#else
+#include <openthread-config.h>
+#endif
+
 #include <stdio.h>
 #include <stdlib.h>
 #include "utils/wrap_string.h"
diff --git a/tests/unit/test_strnlen.c b/tests/unit/test_strnlen.c
index df2490d98..cd6dde00b 100644
--- a/tests/unit/test_strnlen.c
+++ b/tests/unit/test_strnlen.c
@@ -25,6 +25,12 @@
  *    SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#ifdef OPENTHREAD_CONFIG_FILE
+#include OPENTHREAD_CONFIG_FILE
+#else
+#include <openthread-config.h>
+#endif
+
 #include <stdio.h>
 #include <stdlib.h>
 #include "utils/wrap_string.h"