diff --git a/index.html b/index.html
index 9ee371086..f0ad16f7a 100644
--- a/index.html
+++ b/index.html
@@ -12,7 +12,7 @@
   <body>
     <button class="back-to-top-button" aria-label="back to top">↑</button>
     <nav class="sidebar" aria-label="Table of Contents">
-      <button class="hamburger hamburger--spin sidebar__menu" type="button" aria-label="Menu">
+      <button class="hamburger hamburger--spin sidebar__menu" type="button" aria-label="Menu" aria-expanded="false">
         <span class="hamburger-box">
           <span class="hamburger-inner"></span>
         </span>
diff --git a/src/js/components/Menu.js b/src/js/components/Menu.js
index 6bf3ee814..9634c65aa 100644
--- a/src/js/components/Menu.js
+++ b/src/js/components/Menu.js
@@ -5,7 +5,11 @@ const menu = select('.hamburger')
 const links = select('.sidebar__links')
 const ACTIVE_CLASS = 'is-active'
 
-const toggle = () => [menu, links].forEach(el => el.classList.toggle(ACTIVE_CLASS))
+const toggle = () => {
+  const els = [menu, links]
+  els.forEach(el => el.classList.toggle(ACTIVE_CLASS))
+  menu.setAttribute('aria-expanded', menu.classList.contains(ACTIVE_CLASS) ? 'true' : 'false')
+}
 
 menu.addEventListener('click', toggle)
 
