diff --git a/crates/mdbook-html/front-end/templates/toc.js.hbs b/crates/mdbook-html/front-end/templates/toc.js.hbs
index 1a9f751ccf..d5be5b1189 100644
--- a/crates/mdbook-html/front-end/templates/toc.js.hbs
+++ b/crates/mdbook-html/front-end/templates/toc.js.hbs
@@ -23,7 +23,8 @@ class MDBookSidebarScrollbox extends HTMLElement {
                 link.href = path_to_root + href;
             }
             // The 'index' page is supposed to alias the first chapter in the book.
-            if (link.href === current_page
+            // Check both with and without the '.html' suffix to be robust against pretty URLs
+            if (link.href.replace(/\.html$/, '') === current_page.replace(/\.html$/, '')
                 || i === 0
                 && path_to_root === ''
                 && current_page.endsWith('/index.html')) {