diff --git a/format.c b/format.c
index 275d5218..f69330b2 100644
--- a/format.c
+++ b/format.c
@@ -2077,6 +2077,18 @@ format_cb_scroll_region_upper(struct format_tree *ft)
 	return (NULL);
 }
 
+/* Callback for server_sessions. */
+static void *
+format_cb_server_sessions(__unused struct format_tree *ft)
+{
+	struct session	*s;
+	u_int		 n = 0;
+
+	RB_FOREACH(s, sessions, &sessions)
+		n++;
+	return (format_printf("%u", n));
+}
+
 /* Callback for session_attached. */
 static void *
 format_cb_session_attached(struct format_tree *ft)
@@ -2980,6 +2992,9 @@ static const struct format_table_entry format_table[] = {
 	{ "scroll_region_upper", FORMAT_TABLE_STRING,
 	  format_cb_scroll_region_upper
 	},
+	{ "server_sessions", FORMAT_TABLE_STRING,
+	  format_cb_server_sessions
+	},
 	{ "session_activity", FORMAT_TABLE_TIME,
 	  format_cb_session_activity
 	},
diff --git a/tmux.1 b/tmux.1
index 92e2915f..7bf2bc4d 100644
--- a/tmux.1
+++ b/tmux.1
@@ -5278,6 +5278,7 @@ The following variables are available, where appropriate:
 .It Li "selection_present" Ta "" Ta "1 if selection started in copy mode"
 .It Li "selection_start_x" Ta "" Ta "X position of the start of the selection"
 .It Li "selection_start_y" Ta "" Ta "Y position of the start of the selection"
+.It Li "server_sessions" Ta "" Ta "Number of sessions"
 .It Li "session_activity" Ta "" Ta "Time of session last activity"
 .It Li "session_alerts" Ta "" Ta "List of window indexes with alerts"
 .It Li "session_attached" Ta "" Ta "Number of clients session is attached to"