diff --git a/bits/types/once_flag.h b/bits/types/once_flag.h
new file mode 100644
index 0000000000..9180adce1a
--- /dev/null
+++ b/bits/types/once_flag.h
@@ -0,0 +1,27 @@
+/* Define once_flag and ONCE_FLAG_INIT.
+   Copyright (C) 2018-2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef __once_flag_defined
+#define __once_flag_defined 1
+
+#include <bits/thread-shared-types.h>
+
+typedef __once_flag once_flag;
+#define ONCE_FLAG_INIT __ONCE_FLAG_INIT
+
+#endif
diff --git a/manual/threads.texi b/manual/threads.texi
index eeefa9b800..1e9be32d66 100644
--- a/manual/threads.texi
+++ b/manual/threads.texi
@@ -219,6 +219,9 @@ invoked from several threads.  The completion of the function
 @code{call_once} with the same @code{flag} variable.
 @end deftypefun
 
+These definitions are from C11, where they appear in @file{threads.h}.
+In C23, they appear in @file{stdlib.h} as well as in @file{threads.h}.
+
 
 @node ISO C Mutexes
 @subsection Mutexes
diff --git a/stdlib/Makefile b/stdlib/Makefile
index 25f777e1a5..d2c594885a 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -37,6 +37,7 @@ headers := \
   bits/time64.h \
   bits/timesize.h \
   bits/types/error_t.h \
+  bits/types/once_flag.h \
   bits/wordsize.h \
   errno.h \
   fmtmsg.h \
diff --git a/stdlib/stdlib.h b/stdlib/stdlib.h
index cd4503c761..1d6a83a22c 100644
--- a/stdlib/stdlib.h
+++ b/stdlib/stdlib.h
@@ -1158,6 +1158,14 @@ extern int getloadavg (double __loadavg[], int __nelem)
 extern int ttyslot (void) __THROW;
 #endif
 
+#if __GLIBC_USE (ISOC23)
+# include <bits/types/once_flag.h>
+
+/* Call function __FUNC exactly once, even if invoked from several threads.
+   All calls must be made with the same __FLAGS object.  */
+extern void call_once (once_flag *__flag, void (*__func)(void));
+#endif
+
 #include <bits/stdlib-float.h>
 
 /* Define some macros helping to catch buffer overflows.  */
diff --git a/sysdeps/pthread/threads.h b/sysdeps/pthread/threads.h
index 44c9bc7b1f..48fcda5992 100644
--- a/sysdeps/pthread/threads.h
+++ b/sysdeps/pthread/threads.h
@@ -25,6 +25,7 @@
 __BEGIN_DECLS
 
 #include <bits/thread-shared-types.h>
+#include <bits/types/once_flag.h>
 #include <bits/types/struct_timespec.h>
 
 #if (!defined __STDC_VERSION__				\
@@ -58,9 +59,6 @@ enum
   mtx_timed     = 2
 };
 
-typedef __once_flag once_flag;
-#define ONCE_FLAG_INIT __ONCE_FLAG_INIT
-
 typedef union
 {
   char __size[__SIZEOF_PTHREAD_MUTEX_T];