diff --git a/tmux.1 b/tmux.1
index 9dd2065e..7cb880ed 100644
--- a/tmux.1
+++ b/tmux.1
@@ -3781,6 +3781,7 @@ The following variables are available, where appropriate:
 .It Li "pane_tty" Ta "" Ta "Pseudo terminal of pane"
 .It Li "pane_width" Ta "" Ta "Width of pane"
 .It Li "pid" Ta ""  Ta "Server PID"
+.It Li "rectangle_toggle" Ta "" Ta "1 if rectangle selection is activated"
 .It Li "scroll_region_lower" Ta "" Ta "Bottom of scroll region in pane"
 .It Li "scroll_region_upper" Ta "" Ta "Top of scroll region in pane"
 .It Li "scroll_position" Ta "" Ta "Scroll position in copy mode"
diff --git a/window-copy.c b/window-copy.c
index 93e7dc69..59c70230 100644
--- a/window-copy.c
+++ b/window-copy.c
@@ -2447,6 +2447,7 @@ window_copy_add_formats(struct window_pane *wp, struct format_tree *ft)
 
 	format_add(ft, "selection_present", "%d", data->screen.sel.flag);
 	format_add(ft, "scroll_position", "%d", data->oy);
+	format_add(ft, "rectangle_toggle", "%d", data->rectflag);
 }
 
 static void