diff --git a/README.md b/README.md
index 076bd0e8..1ed96d1f 100644
--- a/README.md
+++ b/README.md
@@ -68,6 +68,7 @@ GLFW bundles a number of dependencies in the `deps/` directory.
  - [X11] Added support for Cygwin-X
  - [X11] Made XInput2 optional at compile-time
  - [X11] Made Xxf86vm optional at compile-time
+ - [X11] Bugfix: Mouse button `GLFW_MOUSE_BUTTON_4` was never used
  - [X11] Bugfix: `glfwTerminate` could close an unrelated file descriptor
  - [X11] Bugfix: Some WMs did not respect cursor redefinition
 
diff --git a/src/x11_window.c b/src/x11_window.c
index 950078fa..f30cf6f8 100644
--- a/src/x11_window.c
+++ b/src/x11_window.c
@@ -1011,7 +1011,7 @@ static void processEvent(XEvent *event)
                 // Additional buttons after 7 are treated as regular buttons
                 // We subtract 4 to fill the gap left by scroll input above
                 _glfwInputMouseClick(window,
-                                     event->xbutton.button - 4,
+                                     event->xbutton.button - Button1 - 4,
                                      GLFW_PRESS,
                                      mods);
             }