diff --git a/Misc/ACKS b/Misc/ACKS
index c1c29d2ea1d..89751332a90 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -52,6 +52,7 @@ Ankur Ankan
 Jon Anglin
 Heidi Annexstad
 Ramchandra Apte
+Arfrever Frehtes Taifersar Arahesis
 Éric Araujo
 Alicia Arlen
 Jeffrey Armstrong
diff --git a/Misc/NEWS b/Misc/NEWS
index 851e9258d62..fa36c6b8a57 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -113,6 +113,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21704: Fix build error for _multiprocessing when semaphores
+  are not available.  Patch by Arfrever Frehtes Taifersar Arahesis.
+
 - Issue #20173: Convert sha1, sha256, sha512 and md5 to ArgumentClinic.
   Patch by Vajrasky Kok.
 
diff --git a/Modules/_multiprocessing/multiprocessing.c b/Modules/_multiprocessing/multiprocessing.c
index 1aaf3605714..4ae638eea5c 100644
--- a/Modules/_multiprocessing/multiprocessing.c
+++ b/Modules/_multiprocessing/multiprocessing.c
@@ -128,7 +128,9 @@ static PyMethodDef module_methods[] = {
     {"recv", multiprocessing_recv, METH_VARARGS, ""},
     {"send", multiprocessing_send, METH_VARARGS, ""},
 #endif
+#ifndef POSIX_SEMAPHORES_NOT_ENABLED
     {"sem_unlink", _PyMp_sem_unlink, METH_VARARGS, ""},
+#endif
     {NULL}
 };