diff --git a/src/utest/mailbox_tc.c b/src/utest/mailbox_tc.c
index b51ee6e515e..6d7ce132878 100644
--- a/src/utest/mailbox_tc.c
+++ b/src/utest/mailbox_tc.c
@@ -389,6 +389,17 @@ static void test_dynamic_mailbox_send_recv(void)
 
 static rt_err_t utest_tc_init(void)
 {
+    /* This testcase may be run more than once in a single boot. The finish
+     * flags are set by the worker threads and read by the test body, so they
+     * must be cleared before every run. Otherwise the second run would skip
+     * the wait loop, detach/delete the mailbox while the worker threads are
+     * still blocked in rt_mb_recv(), and those threads would then use an
+     * already detached mailbox. */
+    static_mb_recv_thread_finish = 0;
+    static_mb_send_thread_finish = 0;
+    dynamic_mb_recv_thread_finish = 0;
+    dynamic_mb_send_thread_finish = 0;
+
     return RT_EOK;
 }