diff --git a/.eleventy.js b/.eleventy.js
index 01105ef345..94f317a883 100644
--- a/.eleventy.js
+++ b/.eleventy.js
@@ -141,6 +141,7 @@ module.exports = function(eleventyConfig) {
     eleventyConfig.addPassthroughCopy("src/js/ai-expert-modal.js");
     eleventyConfig.addPassthroughCopy("src/js/hm-promo-banner.js");
     eleventyConfig.addPassthroughCopy("src/js/nav-tracking.js");
+    eleventyConfig.addPassthroughCopy("src/js/signup-popup.js");
 
     // Watch content images for the image pipeline
     eleventyConfig.addWatchTarget("src/**/*.{svg,webp,png,jpeg,gif}");
diff --git a/nuxt/nuxt.config.ts b/nuxt/nuxt.config.ts
index 9ec6ed4072..bcc399b859 100644
--- a/nuxt/nuxt.config.ts
+++ b/nuxt/nuxt.config.ts
@@ -291,6 +291,9 @@ export default defineNuxtConfig({
                 // Explicit nav-click tracking. Source is src/js/nav-tracking.js;
                 // prod:eleventy-nuxt copies the 11ty output into nuxt/public/.
                 { src: '/js/nav-tracking.js', defer: true },
+                // Opens sign-up in a small popup window on desktop/tablet.
+                // Source is src/js/signup-popup.js; copied the same way as nav-tracking.js.
+                { src: '/js/signup-popup.js', defer: true },
             ]
         }
     },
diff --git a/src/_includes/layouts/base.njk b/src/_includes/layouts/base.njk
index bf62556b3b..12a5426a8e 100644
--- a/src/_includes/layouts/base.njk
+++ b/src/_includes/layouts/base.njk
@@ -158,6 +158,8 @@ eleventyComputed:
     <script type="module" src="/js/flowrenderer.min.js"></script>
     <!-- Explicit nav-click tracking; keep in sync with the Nuxt copy -->
     <script defer src="/js/nav-tracking.js"></script>
+    <!-- Opens sign-up in a small popup window on desktop/tablet -->
+    <script defer src="/js/signup-popup.js"></script>
 
     {%- if not DEV_MODE -%}
     {% include "analytics/head.html" %}
diff --git a/src/js/signup-popup.js b/src/js/signup-popup.js
new file mode 100644
index 0000000000..4e1d77959a
--- /dev/null
+++ b/src/js/signup-popup.js
@@ -0,0 +1,115 @@
+// Opens the sign-up page in a small popup window on desktop/tablet, centered
+// over the visible page content, with a dark overlay behind it.
+// On mobile, leaves the link's default navigation untouched.
+document.addEventListener('DOMContentLoaded', () => {
+    // Tracks the one popup/overlay pair that can be open at a time, so
+    // there's a single thing to clean up instead of separate mutable
+    // variables that a second click could overwrite and orphan.
+    let session = null
+
+    function closeSession () {
+        if (!session) {
+            return
+        }
+        if (!session.popup.closed) {
+            session.popup.close()
+        }
+        clearInterval(session.pollClosed)
+        session.overlay.remove()
+        session = null
+    }
+
+    function openPopup (href) {
+        if (session) {
+            if (!session.popup.closed) {
+                // Already have one open - bring it forward instead of
+                // opening a second popup and orphaning this one's overlay.
+                session.popup.focus()
+                return
+            }
+            closeSession()
+        }
+
+        const width = 420
+        const height = Math.min(900, window.innerHeight * 0.75)
+
+        // outerWidth/outerHeight include browser chrome (toolbars, a
+        // vertical tab strip, etc). Subtracting innerWidth/innerHeight
+        // estimates that chrome so we can center over the visible page
+        // content instead of the full browser window.
+        const chromeWidth = window.outerWidth - window.innerWidth
+        const chromeHeight = window.outerHeight - window.innerHeight
+        const viewportLeft = window.screenX + chromeWidth
+        const viewportTop = window.screenY + chromeHeight
+
+        const left = viewportLeft + (window.innerWidth - width) / 2
+        const top = viewportTop + (window.innerHeight - height) / 2
+
+        const popup = window.open(
+            href,
+            'flowfuse-signup',
+            `width=${width},height=${height},left=${left},top=${top},menubar=no,toolbar=no,location=no,status=no`
+        )
+
+        if (!popup) {
+            return
+        }
+
+        const overlay = document.createElement('div')
+        overlay.setAttribute('id', 'signup-popup-overlay')
+        overlay.style.position = 'fixed'
+        overlay.style.inset = '0'
+        overlay.style.background = 'rgba(0, 0, 0, 0.45)'
+        overlay.style.zIndex = '9999'
+        overlay.addEventListener('click', closeSession)
+        document.body.appendChild(overlay)
+
+        const pollClosed = setInterval(() => {
+            if (popup.closed) {
+                closeSession()
+            }
+        }, 100)
+
+        session = { popup, overlay, pollClosed }
+        popup.focus()
+    }
+
+    // Delegated on `document` (rather than binding each matching link once)
+    // so this also catches anchors rendered after this script ran - e.g.
+    // Nuxt's client-side route navigation swapping in a new CtaSignUp link
+    // without a full page load.
+    document.addEventListener('click', (event) => {
+        const link = event.target.closest('a[href*="/account/create"]')
+        if (!link) {
+            return
+        }
+
+        const isDesktopOrTablet = window.matchMedia('(min-width: 768px)').matches
+        if (!isDesktopOrTablet) {
+            return
+        }
+
+        event.preventDefault()
+
+        // Explicit signal for the product to key its popup-specific
+        // layout off, instead of `window.opener` — that's also set for
+        // an ordinary ctrl/cmd-click "open in new tab", which isn't
+        // this popup at all.
+        const popupUrl = new URL(link.href)
+        popupUrl.searchParams.set('context', 'popup')
+
+        openPopup(popupUrl.href)
+    })
+
+    // Deliberate trade-off, not an oversight: closing on any window focus
+    // (e.g. switching tabs/apps and coming back) can cancel an in-progress,
+    // not-yet-submitted sign-up. That's judged preferable to the
+    // alternative - the popup falling behind this window with no auto-close,
+    // which leaves the overlay covering the page with no visible popup and
+    // no clear reason why, reading as the site being stuck rather than a
+    // sign-up that's still open one window over. Nothing of substance is
+    // lost by closing early here: this only ever covers the initial,
+    // unsubmitted form step. Added once (not per click) so listeners don't
+    // accumulate across retries.
+    window.addEventListener('focus', closeSession)
+})