diff --git a/packages/@headlessui-vue/src/components/portal/portal.test.ts b/packages/@headlessui-vue/src/components/portal/portal.test.ts
index e333c72dd5..6c73530ab8 100644
--- a/packages/@headlessui-vue/src/components/portal/portal.test.ts
+++ b/packages/@headlessui-vue/src/components/portal/portal.test.ts
@@ -3,6 +3,7 @@ import { renderToString } from 'vue/server-renderer'
 import { html } from '../../test-utils/html'
 import { click } from '../../test-utils/interactions'
 import { createRenderTemplate } from '../../test-utils/vue-testing-library'
+import { TransitionRoot } from '../transitions/transition'
 import { Portal, PortalGroup } from './portal'
 
 function getPortalRoot() {
@@ -22,7 +23,7 @@ beforeAll(() => {
 
 afterAll(() => jest.restoreAllMocks())
 
-const renderTemplate = createRenderTemplate({ Portal, PortalGroup })
+const renderTemplate = createRenderTemplate({ Portal, PortalGroup, TransitionRoot })
 
 async function ssrRenderTemplate(input: string | ComponentOptionsWithoutProps) {
   let defaultComponents = { Portal, PortalGroup }
@@ -436,3 +437,40 @@ it('the root shared by multiple portals should not unmount when they change in t
   // The portal root is gone because there are no visible portals
   expect(root()).toBe(null)
 })
+
+it('should render portal content synchronously so that transitions can apply enter classes', async () => {
+  // Regression test for https://github.com/tailwindlabs/headlessui/issues/3456
+  //
+  // When the portal defers rendering to `onMounted`, the Teleport content
+  // appears one tick after the Transition's enter phase has started. The
+  // enter-from classes never get applied, so the transition is invisible.
+  renderTemplate({
+    template: html`
+      <TransitionRoot :show="show" enter="enter" enterFrom="enter-from" enterTo="enter-to">
+        <Portal>
+          <div id="transitioned">Hello</div>
+        </Portal>
+      </TransitionRoot>
+
+      <button id="toggle" @click="show = !show">Toggle</button>
+    `,
+    setup() {
+      let show = ref(false)
+      return { show }
+    },
+  })
+
+  // Portal root shouldn't exist yet (nothing shown)
+  expect(getPortalRoot()).toBe(null)
+
+  // Toggle show to true — the portal content should render and the
+  // transition should apply enter-from classes in the same tick
+  await click(document.getElementById('toggle'))
+
+  // The portal should have rendered
+  let content = document.getElementById('transitioned')
+  expect(content).not.toBe(null)
+
+  // The portal root should exist
+  expect(getPortalRoot()).not.toBe(null)
+})
diff --git a/packages/@headlessui-vue/src/components/portal/portal.ts b/packages/@headlessui-vue/src/components/portal/portal.ts
index dc09821fa1..ae4b6bb84d 100644
--- a/packages/@headlessui-vue/src/components/portal/portal.ts
+++ b/packages/@headlessui-vue/src/components/portal/portal.ts
@@ -18,6 +18,7 @@ import {
 } from 'vue'
 import { usePortalRoot } from '../../internal/portal-force-root'
 import { dom } from '../../utils/dom'
+import { env } from '../../utils/env'
 import { getOwnerDocument } from '../../utils/owner'
 import { render } from '../../utils/render'
 
@@ -89,7 +90,17 @@ export let Portal = defineComponent({
       setCount(myTarget.value, (val) => val + 1)
     }
 
-    let ready = ref(false)
+    // Start ready on the client so that the portal renders synchronously
+    // with the component tree. Deferring to `onMounted` causes a one-tick
+    // delay that races against Vue 3.5+'s Transition lifecycle — the
+    // enter-from classes never get applied because the element appears
+    // after the transition's enter phase has already started.
+    //
+    // On the server we must remain `false` to avoid SSR hydration
+    // mismatches (portals can't render server-side).
+    //
+    // Fixes: https://github.com/tailwindlabs/headlessui/issues/3456
+    let ready = ref(env.isClient)
     onMounted(() => {
       ready.value = true
     })