diff --git a/format.c b/format.c
index f1f52895..f0a5a3b1 100644
--- a/format.c
+++ b/format.c
@@ -2529,6 +2529,11 @@ format_defaults_session(struct format_tree *ft, struct session *s)
 
 	format_add_cb(ft, "session_alerts", format_cb_session_alerts);
 	format_add_cb(ft, "session_stack", format_cb_session_stack);
+
+	if (server_check_marked() && marked_pane.s == s)
+	    format_add(ft, "session_marked", "1");
+	else
+	    format_add(ft, "session_marked", "0");
 }
 
 /* Set default format keys for a client. */
diff --git a/tmux.1 b/tmux.1
index b99fcb62..960d31d9 100644
--- a/tmux.1
+++ b/tmux.1
@@ -4564,6 +4564,7 @@ The following variables are available, where appropriate:
 .It Li "session_id" Ta "" Ta "Unique session ID"
 .It Li "session_last_attached" Ta "" Ta "Time session last attached"
 .It Li "session_many_attached" Ta "" Ta "1 if multiple clients attached"
+.It Li "session_marked" Ta "" Ta "1 if this session contains the marked pane"
 .It Li "session_name" Ta "#S" Ta "Name of session"
 .It Li "session_path" Ta "" Ta "Working directory of session"
 .It Li "session_stack" Ta "" Ta "Window indexes in most recent order"