diff --git a/tmux.1 b/tmux.1
index 5bf5b1e7..dc78abdc 100644
--- a/tmux.1
+++ b/tmux.1
@@ -4351,6 +4351,7 @@ The following variables are available, where appropriate:
 .It Li "scroll_position" Ta "" Ta "Scroll position in copy mode"
 .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 "selection_active" Ta "" Ta "1 if selection started and changes with the cursor in copy mode"
 .It Li "selection_end_x" Ta "" Ta "X position of the end of the selection"
 .It Li "selection_end_y" Ta "" Ta "Y position of the end of the selection"
 .It Li "selection_present" Ta "" Ta "1 if selection started in copy mode"
diff --git a/window-copy.c b/window-copy.c
index a717884c..301fc16e 100644
--- a/window-copy.c
+++ b/window-copy.c
@@ -590,7 +590,10 @@ window_copy_formats(struct window_mode_entry *wme, struct format_tree *ft)
 		format_add(ft, "selection_start_y", "%d", data->sely);
 		format_add(ft, "selection_end_x", "%d", data->endselx);
 		format_add(ft, "selection_end_y", "%d", data->endsely);
-	}
+		format_add(ft, "selection_active", "%d",
+		    data->cursordrag != CURSORDRAG_NONE);
+	} else
+		format_add(ft, "selection_active", "%d", 0);
 
 	s = format_grid_word(data->screen.grid, data->cx, data->cy);
 	if (s != NULL) {