diff --git a/.changeset/lit-controller-set-options.md b/.changeset/lit-controller-set-options.md
new file mode 100644
index 000000000..589c36069
--- /dev/null
+++ b/.changeset/lit-controller-set-options.md
@@ -0,0 +1,17 @@
+---
+'@tanstack/lit-virtual': minor
+---
+
+Add `setOptions` to `VirtualizerController` and `WindowVirtualizerController` so virtualizer options can be updated after construction.
+
+Previously the controller only accepted options once, in its constructor, so options derived from reactive Lit properties (`count`, `estimateSize`, `horizontal`, …) could never be re-applied. `getVirtualizer().setOptions(...)` is not a substitute: it replaces options wholesale and drops the controller's `onChange` wrapper, which is what asks the host to re-render.
+
+`setOptions` takes a partial set of options, merges them over the options the controller already holds (skipping `undefined`, so an omitted value does not wipe a stored one), re-applies the host `requestUpdate` wrapper around `onChange`, and flushes the change:
+
+```ts
+willUpdate(changed: PropertyValues<this>) {
+  if (changed.has('count')) {
+    this.virtualizerController.setOptions({ count: this.count })
+  }
+}
+```
diff --git a/docs/framework/lit/lit-virtual.md b/docs/framework/lit/lit-virtual.md
index 8700490a4..98df3c8c3 100644
--- a/docs/framework/lit/lit-virtual.md
+++ b/docs/framework/lit/lit-virtual.md
@@ -34,3 +34,28 @@ private windowVirtualizerController = new WindowVirtualizerController<TItemEleme
 ```
 
 This class stands of window-based `Virtualizer` instance configured to work with an HTML element as the scrollElement.
+
+## Updating options
+
+The controller reads options once, in its constructor. When a reactive property changes, pass the new values to `setOptions` from `willUpdate` (before render). `VirtualizerController` and `WindowVirtualizerController` both implement it.
+
+`setOptions` merges that partial over the options the controller already holds, including its defaults, and keeps the `onChange` wrapper that re-renders the host. Keys set to `undefined` are ignored, so a partial update does not erase an option from an earlier call.
+
+```ts
+@property({ type: Number })
+count = 1000
+
+private virtualizerController = new VirtualizerController(this, {
+  getScrollElement: () => this.scrollElementRef.value,
+  count: this.count,
+  estimateSize: () => 35,
+})
+
+protected willUpdate(changed: PropertyValues<this>) {
+  if (changed.has('count')) {
+    this.virtualizerController.setOptions({ count: this.count })
+  }
+}
+```
+
+Call `setOptions` on the controller. `getVirtualizer().setOptions(...)` replaces options wholesale, drops the host `onChange` wrapper, and leaves the controller's stored options stale.
diff --git a/packages/lit-virtual/README.md b/packages/lit-virtual/README.md
index a16ac0e79..3f3ef0aef 100644
--- a/packages/lit-virtual/README.md
+++ b/packages/lit-virtual/README.md
@@ -59,6 +59,18 @@ class MyVirtualElement extends LitElement {
 
 Note that a [Ref](https://lit.dev/docs/templates/directives/#ref) is attached to the scrolling container to allow the virtualizer to interact with it.
 
+Options passed to the constructor are captured once. When a reactive property changes, merge the new values in `willUpdate` with `setOptions`. The same method exists on `WindowVirtualizerController`.
+
+```ts
+protected willUpdate(changed: PropertyValues<this>) {
+  if (changed.has('count')) {
+    this.virtualizerController.setOptions({ count: this.count })
+  }
+}
+```
+
+`setOptions` merges a partial update over the controller's current options and keeps the `onChange` wrapper that re-renders the host. `undefined` values are ignored. Use the controller method rather than `getVirtualizer().setOptions(...)`, which replaces options wholesale and drops that wrapper.
+
 ## `WindowVirtualizerController`
 
 You can also create a virtualizer controller that attaches to the Window:
diff --git a/packages/lit-virtual/src/index.ts b/packages/lit-virtual/src/index.ts
index 8cfa1d643..37804f623 100644
--- a/packages/lit-virtual/src/index.ts
+++ b/packages/lit-virtual/src/index.ts
@@ -20,38 +20,106 @@ class VirtualizerControllerBase<
 
   private readonly virtualizer: Virtualizer<TScrollElement, TItemElement>
 
+  private options: VirtualizerOptions<TScrollElement, TItemElement>
+
   private cleanup: () => void = () => {}
 
+  /**
+   * True between `hostUpdate` and `hostUpdated`. `willUpdate` runs in that
+   * window, before render, so applying options there is already part of the
+   * current update.
+   */
+  private hostIsUpdating = false
+
   constructor(
     host: ReactiveControllerHost,
     options: VirtualizerOptions<TScrollElement, TItemElement>,
   ) {
-    const resolvedOptions: VirtualizerOptions<TScrollElement, TItemElement> = {
-      ...options,
-      onChange: (instance, sync) => {
-        this.host.updateComplete.then(() => this.host.requestUpdate())
-        options.onChange?.(instance, sync)
-      },
-    }
-    this.virtualizer = new Virtualizer(resolvedOptions)
-    ;(this.host = host).addController(this)
+    this.host = host
+    this.options = { ...options }
+    this.virtualizer = new Virtualizer(this.resolveOptions(this.options))
+    this.host.addController(this)
   }
 
   public getVirtualizer() {
     return this.virtualizer
   }
 
+  /**
+   * Merge a partial options update into the options this controller already
+   * holds, push them to the virtualizer, and re-render the host when this is
+   * called outside an update.
+   *
+   * Keys set to `undefined` are ignored, matching core `setOptions`, so a
+   * partial update cannot wipe an option stored earlier. Defaults supplied by
+   * `VirtualizerController` / `WindowVirtualizerController` stay in place
+   * unless a later call replaces them.
+   */
+  public setOptions(
+    options: Partial<VirtualizerOptions<TScrollElement, TItemElement>>,
+  ) {
+    if (!this.applyOptions(options)) return
+
+    this.virtualizer.setOptions(this.resolveOptions(this.options))
+    this.virtualizer._willUpdate()
+
+    if (!this.hostIsUpdating) {
+      this.host.requestUpdate()
+    }
+  }
+
+  hostUpdate() {
+    this.hostIsUpdating = true
+  }
+
   hostConnected() {
     this.cleanup = this.virtualizer._didMount()
   }
 
   hostUpdated() {
+    this.hostIsUpdating = false
     this.virtualizer._willUpdate()
   }
 
   hostDisconnected() {
     this.cleanup()
   }
+
+  private applyOptions(
+    partial: Partial<VirtualizerOptions<TScrollElement, TItemElement>>,
+  ): boolean {
+    const next: VirtualizerOptions<TScrollElement, TItemElement> = {
+      ...this.options,
+    }
+    let changed = false
+
+    for (const key in partial) {
+      if (!Object.prototype.hasOwnProperty.call(partial, key)) continue
+
+      const value = partial[key as keyof typeof partial]
+      if (value === undefined) continue
+
+      if (!Object.is(next[key as keyof typeof next], value)) {
+        changed = true
+        Object.assign(next, { [key]: value })
+      }
+    }
+
+    if (changed) this.options = next
+    return changed
+  }
+
+  private resolveOptions(
+    options: VirtualizerOptions<TScrollElement, TItemElement>,
+  ): VirtualizerOptions<TScrollElement, TItemElement> {
+    return {
+      ...options,
+      onChange: (instance, sync) => {
+        this.host.updateComplete.then(() => this.host.requestUpdate())
+        options.onChange?.(instance, sync)
+      },
+    }
+  }
 }
 
 export class VirtualizerController<
diff --git a/packages/lit-virtual/tests/set-options.test.ts b/packages/lit-virtual/tests/set-options.test.ts
new file mode 100644
index 000000000..88015e7e2
--- /dev/null
+++ b/packages/lit-virtual/tests/set-options.test.ts
@@ -0,0 +1,201 @@
+import { expect, test } from 'vitest'
+import { elementUpdated, fixture, html } from '@open-wc/testing'
+import { LitElement, type PropertyValues } from 'lit'
+import { customElement, property } from 'lit/decorators.js'
+import type { ReactiveControllerHost } from 'lit'
+import {
+  VirtualizerController,
+  WindowVirtualizerController,
+  observeWindowRect,
+  windowScroll,
+} from '../src'
+
+function createHost() {
+  let requestUpdateCount = 0
+
+  const host: ReactiveControllerHost = {
+    addController() {},
+    removeController() {},
+    requestUpdate() {
+      requestUpdateCount++
+    },
+    get updateComplete() {
+      return Promise.resolve(true)
+    },
+  }
+
+  return {
+    host,
+    get requestUpdateCount() {
+      return requestUpdateCount
+    },
+  }
+}
+
+test('constructor options reach the virtualizer', () => {
+  const { host } = createHost()
+  const estimateSize = () => 50
+  const controller = new VirtualizerController(host, {
+    count: 10,
+    estimateSize,
+    getScrollElement: () => null,
+  })
+
+  const virtualizer = controller.getVirtualizer()
+  expect(virtualizer.options.count).toBe(10)
+  expect(virtualizer.options.estimateSize).toBe(estimateSize)
+  expect(virtualizer.getTotalSize()).toBe(500)
+})
+
+test('setOptions merges partial updates and ignores undefined', () => {
+  const { host } = createHost()
+  const estimateSize = () => 50
+  const controller = new VirtualizerController(host, {
+    count: 10,
+    estimateSize,
+    getScrollElement: () => null,
+  })
+  const virtualizer = controller.getVirtualizer()
+
+  controller.setOptions({ count: 20, estimateSize: undefined })
+
+  expect(virtualizer.options.count).toBe(20)
+  expect(virtualizer.options.estimateSize).toBe(estimateSize)
+  expect(virtualizer.getTotalSize()).toBe(1000)
+
+  const nextEstimate = () => 25
+  controller.setOptions({ estimateSize: nextEstimate, paddingEnd: 12 })
+  expect(virtualizer.options.count).toBe(20)
+  expect(virtualizer.options.estimateSize).toBe(nextEstimate)
+  expect(virtualizer.options.paddingEnd).toBe(12)
+  expect(virtualizer.getTotalSize()).toBe(1012)
+})
+
+test('setOptions keeps the user onChange and the host update wrapper', async () => {
+  const harness = createHost()
+  const calls: Array<boolean> = []
+  const controller = new VirtualizerController(harness.host, {
+    count: 4,
+    estimateSize: () => 25,
+    getScrollElement: () => null,
+    onChange: (_instance, sync) => {
+      calls.push(sync)
+    },
+  })
+
+  controller.setOptions({ count: 6 })
+  controller
+    .getVirtualizer()
+    .options.onChange?.(controller.getVirtualizer(), true)
+
+  expect(calls).toEqual([true])
+  expect(controller.getVirtualizer().options.count).toBe(6)
+  expect(controller.getVirtualizer().getTotalSize()).toBe(150)
+
+  await Promise.resolve()
+  expect(harness.requestUpdateCount).toBeGreaterThan(0)
+})
+
+test('setOptions requests a host update outside an update, and not again for the same options', async () => {
+  const harness = createHost()
+  const controller = new VirtualizerController(harness.host, {
+    count: 10,
+    estimateSize: () => 50,
+    getScrollElement: () => null,
+  })
+
+  controller.setOptions({ count: 12 })
+  await Promise.resolve()
+  const afterChange = harness.requestUpdateCount
+  expect(afterChange).toBeGreaterThan(0)
+
+  controller.setOptions({ count: 12 })
+  await Promise.resolve()
+  expect(harness.requestUpdateCount).toBe(afterChange)
+})
+
+test('setOptions during a host update does not schedule another update', () => {
+  const harness = createHost()
+  const controller = new VirtualizerController(harness.host, {
+    count: 10,
+    estimateSize: () => 50,
+    getScrollElement: () => null,
+  })
+
+  controller.hostUpdate()
+  const before = harness.requestUpdateCount
+  controller.setOptions({ count: 15 })
+  controller.setOptions({
+    getScrollElement: () => null,
+  })
+  expect(controller.getVirtualizer().options.count).toBe(15)
+  expect(controller.getVirtualizer().getTotalSize()).toBe(750)
+  expect(harness.requestUpdateCount).toBe(before)
+  controller.hostUpdated()
+})
+
+test('WindowVirtualizerController.setOptions keeps the window defaults', () => {
+  const { host } = createHost()
+  const controller = new WindowVirtualizerController(host, {
+    count: 2,
+    estimateSize: () => 10,
+  })
+  controller.hostConnected()
+
+  controller.setOptions({ count: 4 })
+
+  const options = controller.getVirtualizer().options
+  expect(options.count).toBe(4)
+  expect(controller.getVirtualizer().getTotalSize()).toBe(40)
+  expect(options.scrollToFn).toBe(windowScroll)
+  expect(options.observeElementRect).toBe(observeWindowRect)
+  expect(options.getScrollElement()).toBe(window)
+
+  controller.hostDisconnected()
+})
+
+@customElement('test-list-set-options')
+class ListSetOptions extends LitElement {
+  @property({ type: Number })
+  count = 10
+
+  private virtualizerController: VirtualizerController<HTMLDivElement, Element>
+
+  constructor() {
+    super()
+    this.virtualizerController = new VirtualizerController(this, {
+      getScrollElement: () => null,
+      count: this.count,
+      estimateSize: () => 50,
+    })
+  }
+
+  protected willUpdate(changed: PropertyValues<this>) {
+    if (changed.has('count')) {
+      this.virtualizerController.setOptions({ count: this.count })
+    }
+  }
+
+  render() {
+    const virtualizer = this.virtualizerController.getVirtualizer()
+    return html`<div data-testid="total">${virtualizer.getTotalSize()}</div>`
+  }
+}
+
+test('a reactive count property updates the rendered total size', async () => {
+  const el = await fixture(
+    html`<test-list-set-options></test-list-set-options>`,
+  )
+  await elementUpdated(el)
+
+  // Raising count does not paint a higher index unless the viewport can
+  // hold it. The rendered total size is what changes: 10 * 50, then 20 * 50.
+  const total = () =>
+    el.shadowRoot!.querySelector('[data-testid="total"]')?.textContent
+
+  expect(total()).toBe('500')
+  ;(el as ListSetOptions).count = 20
+  await elementUpdated(el)
+
+  expect(total()).toBe('1000')
+})