diff --git a/share/man/man9/cdefs.9 b/share/man/man9/cdefs.9
--- a/share/man/man9/cdefs.9
+++ b/share/man/man9/cdefs.9
@@ -3,7 +3,7 @@
 .\"
 .\" SPDX-License-Identifier: BSD-2-Clause
 .\"
-.Dd July 20, 2024
+.Dd February 1, 2025
 .Dt CDEFS 9
 .Os
 .Sh NAME
@@ -80,7 +80,7 @@
 linkage.
 .El
 .Ss Function, Structure and Variable Modifiers
-.Bl -column "---------------"
+.Bl -column "----------------"
 .It Sy Macro Ta Sy Description
 .It Sy __weak_symbol Ta Declare the symbol to be a weak symbol
 .It Sy __dead2 Ta Function will not return
@@ -91,6 +91,7 @@
 .It Sy __deprecated Ta Function interface has been deprecated, and clients
 should migrate to a new interface.
 A warning will be issued for clients of this interface.
+.It Sy __deprecated_msg(m) Ta Same as __deprecated but include a message Fa m Ta to aid migration to new interfaces.
 .It Sy __packed Ta \&Do not have space between structure elements for natural alignment.
 Used when communicating with external protocols.
 .It Sy __aligned(x) Ta Specify in bytes the minimum alignment for the specified field, structure or variable
diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -155,6 +155,7 @@
 #define	__unused	__attribute__((__unused__))
 #define	__used		__attribute__((__used__))
 #define __deprecated	__attribute__((__deprecated__))
+#define __deprecated_msg(m)	__attribute__((__deprecated__(m)))
 #define	__packed	__attribute__((__packed__))
 #define	__aligned(x)	__attribute__((__aligned__(x)))
 #define	__section(x)	__attribute__((__section__(x)))