diff --git a/UPDATING b/UPDATING
index 365972a51542..45a61fc1e9ba 100644
--- a/UPDATING
+++ b/UPDATING
@@ -5,6 +5,17 @@ they are unavoidable.
 You should get into the habit of checking this file for changes each time
 you update your ports collection, before attempting any port upgrades.
 
+20251201:
+  AFFECTS: users of sysutils/tmux
+  AUTHOR: jrm@FreeBSD.org
+
+  tmux must be restarted after upgrading.  This is particularly
+  important with version 3.6 because one of the libraries that tmux uses
+  has changed its protocol.  If you see the following message when you
+  try to attach, it likely means you forgot to restart tmux:
+
+  open terminal failed: not a terminal
+
 20251130:
   AFFECTS: users of www/node{22,24,25}
   AUTHOR: sunpoet@FreeBSD.org
diff --git a/sysutils/tmux/Makefile b/sysutils/tmux/Makefile
index 8c471b440e79..df1538387a6c 100644
--- a/sysutils/tmux/Makefile
+++ b/sysutils/tmux/Makefile
@@ -1,5 +1,6 @@
 PORTNAME=	tmux
 PORTVERSION=	3.6
+PORTREVISION=	1
 CATEGORIES=	sysutils
 MASTER_SITES=	https://github.com/tmux/tmux/releases/download/${PORTVERSION}/
 DISTFILES=	${DISTNAME}${EXTRACT_SUFX}
diff --git a/sysutils/tmux/pkg-message b/sysutils/tmux/pkg-message
new file mode 100644
index 000000000000..8da2da68a602
--- /dev/null
+++ b/sysutils/tmux/pkg-message
@@ -0,0 +1,13 @@
+[
+{
+  type: upgrade
+  message: <<EOM
+tmux must be restarted after upgrading.
+
+If you see the following message when you try to attach, it likely means
+you forgot to restart tmux:
+
+  open terminal failed: not a terminal
+EOM
+}
+]