diff --git a/sys/posix/pthread/include/pthread_once.h b/sys/posix/pthread/include/pthread_once.h
index 6c01a4a1759d..2a10358497d1 100644
--- a/sys/posix/pthread/include/pthread_once.h
+++ b/sys/posix/pthread/include/pthread_once.h
@@ -23,15 +23,21 @@ extern "C" {
 
 /**
  * @brief           Datatype to supply to pthread_once().
+ * @details         This data type must be compatible with the one defined
+ *                  in newlib's `include/sys/_pthreadtypes.h`.
  */
-typedef volatile int pthread_once_t;
+typedef struct {
+  int   is_initialized;  /**< initialized */
+  int   init_executed;   /**< init function executed */
+} pthread_once_t;
 
 /**
  * @def             PTHREAD_ONCE_INIT
  * @brief           Initialization for pthread_once_t.
- * @details         A zeroed out pthread_once_t is initialized.
+ * @details         pthread_once_t variables are declared as initialized, but
+ *                  the init function is not yet executed.
  */
-#define PTHREAD_ONCE_INIT 0
+#define PTHREAD_ONCE_INIT  { 1, 0 }
 
 /**
  * @brief           Helper function that ensures that `init_routine` is called at once.
diff --git a/sys/posix/pthread/pthread_once.c b/sys/posix/pthread/pthread_once.c
index 59bc2d0e7f66..c6ac4dc196fe 100644
--- a/sys/posix/pthread/pthread_once.c
+++ b/sys/posix/pthread/pthread_once.c
@@ -22,11 +22,11 @@
 
 int pthread_once(pthread_once_t *once_control, void (*init_routine)(void))
 {
-    if (*once_control == PTHREAD_ONCE_INIT) {
+    if (!once_control->init_executed) {
         init_routine();
     }
 
-    *once_control = PTHREAD_ONCE_INIT + 1;
+    once_control->init_executed = 1;
 
     return 0;
 }