diff --git a/include/c11/threads_posix.h b/include/c11/threads_posix.h
index 7e96715c248..f9c165df4e0 100644
--- a/include/c11/threads_posix.h
+++ b/include/c11/threads_posix.h
@@ -27,7 +27,9 @@
  * DEALINGS IN THE SOFTWARE.
  */
 #include <stdlib.h>
+#ifndef assert
 #include <assert.h>
+#endif
 #include <limits.h>
 #include <errno.h>
 #include <unistd.h>
diff --git a/include/c11/threads_win32.h b/include/c11/threads_win32.h
index d4c2a72d277..5298a84321d 100644
--- a/include/c11/threads_win32.h
+++ b/include/c11/threads_win32.h
@@ -26,7 +26,9 @@
  * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  * DEALINGS IN THE SOFTWARE.
  */
+#ifndef assert
 #include <assert.h>
+#endif
 #include <limits.h>
 #include <errno.h>
 #include <process.h>  // MSVCRT