diff --git a/user_guide_src/source/_static/js/version_switcher.js b/user_guide_src/source/_static/js/version_switcher.js
new file mode 100644
index 000000000000..a201bb4ddc7a
--- /dev/null
+++ b/user_guide_src/source/_static/js/version_switcher.js
@@ -0,0 +1,119 @@
+/*
+ * Version switcher for the user guide sidebar.
+ *
+ * Injects the sphinx_rtd_theme's native ".switch-menus > .version-switch"
+ * scaffolding above the search box in the left sidebar, containing a <select>
+ * so readers can jump between:
+ *   - stable:               https://codeigniter.com/user_guide/
+ *   - latest (in-progress): https://codeigniter4.github.io/userguide/
+ *   - local build:          file:// or localhost, etc. (only shown when not on a deployed host)
+ */
+(() => {
+	const VERSIONS = [
+		{
+			slug: 'stable',
+			label: 'Stable',
+			host: 'codeigniter.com',
+			pathPrefix: '/user_guide/',
+		},
+		{
+			slug: 'latest (in-progress)',
+			label: 'Latest (in-progress)',
+			host: 'codeigniter4.github.io',
+			pathPrefix: '/userguide/',
+		},
+	];
+
+	const LOCAL = Object.freeze({
+		slug: 'local build',
+		label: 'Local build',
+		host: null,
+		pathPrefix: null,
+	});
+
+	const detect_current = () =>
+		VERSIONS.find((version) => version.host === window.location.hostname) ?? LOCAL;
+
+	/*
+	 * Derive the absolute URL of the documentation root by inspecting the
+	 * <script> tag Sphinx always injects for documentation_options.js. Its
+	 * resolved .src is absolute, so we can back out the directory that
+	 * contains _static/, which is the doc root. This works identically for
+	 * http(s), file://, and localhost.
+	 */
+	const doc_root_url = () => {
+		const opts = document.querySelector('script[src*="documentation_options.js"]');
+
+		if (!opts) {
+			return null;
+		}
+
+		const idx = opts.src.lastIndexOf('/_static/');
+
+		return idx < 0 ? null : opts.src.slice(0, idx + 1);
+	};
+
+	const current_page_path = () => {
+		const root = doc_root_url();
+
+		if (!root) {
+			return '';
+		}
+
+        const here = window.location.href.split('#')[0].split('?')[0];
+
+        return here.startsWith(root) ? here.slice(root.length) : '';
+	};
+
+	const url_for = ({ host, pathPrefix }) =>
+		`https://${host}${pathPrefix}${current_page_path()}${window.location.hash}`;
+
+	const build = () => {
+		const searchArea = document.querySelector('.wy-side-nav-search');
+		const searchForm = searchArea?.querySelector('[role="search"]');
+
+		if (!searchArea || !searchForm) {
+			return;
+		}
+
+        if (searchArea.querySelector('.switch-menus')) {
+			return;
+		}
+
+		const current = detect_current();
+		const entries = current === LOCAL ? [LOCAL, ...VERSIONS] : VERSIONS;
+
+		const select = document.createElement('select');
+		select.setAttribute('aria-label', 'Select documentation version');
+
+		for (const entry of entries) {
+			const option = document.createElement('option');
+			option.value = entry.host ? url_for(entry) : '';
+			option.textContent = entry.label;
+			option.selected = entry.slug === current.slug;
+			select.append(option);
+		}
+
+		select.addEventListener('change', () => {
+			if (select.value) {
+				window.location.href = select.value;
+			}
+		});
+
+		const versionSwitch = document.createElement('div');
+		versionSwitch.className = 'version-switch';
+		versionSwitch.append(select);
+
+		const switchMenus = document.createElement('div');
+		switchMenus.className = 'switch-menus';
+		switchMenus.append(versionSwitch);
+
+		searchArea.insertBefore(switchMenus, searchForm);
+	};
+
+	if (document.readyState === 'loading') {
+		document.addEventListener('DOMContentLoaded', build, { once: true });
+	} else {
+		build();
+	}
+})();
diff --git a/user_guide_src/source/conf.py b/user_guide_src/source/conf.py
index 7fa5ade71d7e..24e23f22aafa 100644
--- a/user_guide_src/source/conf.py
+++ b/user_guide_src/source/conf.py
@@ -111,7 +111,8 @@
 # A list of JS files.
 html_js_files = [
 	'js/citheme.js',
-	'js/carbon.js'
+	'js/carbon.js',
+	'js/version_switcher.js'
 ]
 
 # -- Options for LaTeX output --------------------------------------------------