diff --git a/client/src/controllers/BlockController.test.js b/client/src/controllers/BlockController.test.js
new file mode 100644
index 0000000000..308fb004d3
--- /dev/null
+++ b/client/src/controllers/BlockController.test.js
@@ -0,0 +1,125 @@
+import { Application } from '@hotwired/stimulus';
+import { BlockController } from './BlockController';
+
+const render = jest.fn();
+const unpack = jest.fn(() => ({ render }));
+window.telepath = { unpack };
+
+describe('BlockController', () => {
+  const eventNames = ['w-block:ready'];
+
+  const events = {};
+
+  eventNames.forEach((name) => {
+    document.addEventListener(name, (event) => {
+      events[name].push(event);
+    });
+  });
+
+  let application;
+  let errors = [];
+
+  const setup = (html, { identifier = 'w-block' } = {}) => {
+    document.body.innerHTML = `<main>${html}</main>`;
+
+    application = new Application();
+
+    application.register(identifier, BlockController);
+
+    application.handleError = (error, message) => {
+      errors.push({ error, message });
+    };
+
+    application.start();
+
+    return Promise.resolve();
+  };
+
+  beforeEach(() => {
+    application?.stop();
+    document.body.innerHTML = '';
+    errors = [];
+    eventNames.forEach((name) => {
+      events[name] = [];
+    });
+    jest.clearAllMocks();
+  });
+
+  it('does nothing if block element is not found', async () => {
+    await setup('<div></div>');
+
+    expect(errors).toHaveLength(0);
+
+    expect(unpack).not.toHaveBeenCalled();
+
+    expect(events['w-block:ready']).toHaveLength(0);
+  });
+
+  it('should render block if element is controlled', async () => {
+    const data = { _args: ['...'], _type: 'wagtail.blocks.StreamBlock' };
+
+    await setup(
+      `<div
+        id="my-element"
+        data-controller="w-block"
+        data-w-block-data-value='${JSON.stringify(data)}'
+        >
+      </div>`,
+    );
+
+    expect(errors).toHaveLength(0);
+    expect(unpack).toHaveBeenCalledWith(data);
+    expect(render).toHaveBeenCalledWith(
+      document.getElementById('my-element'),
+      'my-element',
+    );
+    expect(events['w-block:ready']).toHaveLength(1);
+  });
+
+  it('should call the unpacked render function with provided initial & error data', async () => {
+    const data = { _args: ['...'], _type: 'wagtail.blocks.StreamBlock' };
+    const initialData = [{ type: 'paragraph_block', value: '...' }];
+    const errorData = { messages: ['An error...'] };
+
+    await setup(
+      `<div
+        id="my-element"
+        data-controller="w-block"
+        data-w-block-arguments-value='${JSON.stringify([initialData, errorData])}'
+        data-w-block-data-value='${JSON.stringify(data)}'
+        >
+      </div>`,
+    );
+
+    expect(errors).toHaveLength(0);
+    expect(unpack).toHaveBeenCalledWith(data);
+    expect(render).toHaveBeenCalledWith(
+      document.getElementById('my-element'),
+      'my-element',
+      initialData,
+      errorData,
+    );
+    expect(events['w-block:ready']).toHaveLength(1);
+  });
+
+  it('should throw an error if used on an element without an id', async () => {
+    await setup('<div data-controller="w-block"></div>');
+
+    expect(errors).toHaveLength(1);
+    expect(errors).toHaveProperty(
+      '0.error.message',
+      'Controlled element needs an id attribute.',
+    );
+  });
+
+  it('should throw an error if Telepath is not available in the window global', async () => {
+    delete window.telepath;
+    await setup('<div id="my-element" data-controller="w-block"></div>');
+
+    expect(errors).toHaveLength(1);
+    expect(errors).toHaveProperty(
+      '0.error.message',
+      '`window.telepath` is not available.',
+    );
+  });
+});
diff --git a/client/src/controllers/BlockController.ts b/client/src/controllers/BlockController.ts
new file mode 100644
index 0000000000..a240acf382
--- /dev/null
+++ b/client/src/controllers/BlockController.ts
@@ -0,0 +1,86 @@
+import { Controller } from '@hotwired/stimulus';
+
+declare global {
+  interface Window {
+    initBlockWidget?: (id: string) => void;
+    telepath: any;
+  }
+}
+
+/**
+ * Adds the ability to unpack a Telepath object and render it on the controlled element.
+ * Used to initialize the top-level element of a BlockWidget (the form widget for a StreamField).
+ *
+ * @example
+ * <div
+ *  id="some-id"
+ *  data-controller="w-block"
+ *  data-w-block-data-value='{"_args":["..."], "_type": "wagtail.blocks.StreamBlock"}'
+ * >
+ * </div>
+ *
+ * @example - with initial arguments
+ * <div
+ *  id="some-id"
+ *  data-controller="w-block"
+ *  data-w-block-data-value='{"_args":["..."], "_type": "wagtail.blocks.StreamBlock"}'
+ *  data-w-block-arguments-value='[[{ type: "paragraph_block", value: "..."}], {messages:["An error..."]}]'
+ * >
+ * </div>
+ */
+export class BlockController extends Controller<HTMLElement> {
+  static values = {
+    arguments: { type: Array, default: [] },
+    data: { type: Object, default: {} },
+  };
+
+  /** Array of arguments to pass to the render method of the block [initial value, errors]. */
+  declare argumentsValue: Array<string>;
+  /** Block definition to be passed to `telepath.unpack`, used to obtain a JavaScript representation of the block. */
+  declare dataValue: object;
+
+  connect() {
+    const telepath = window.telepath;
+
+    if (!telepath) {
+      throw new Error('`window.telepath` is not available.');
+    }
+
+    const element = this.element;
+    const id = element.id;
+
+    if (!id) {
+      throw new Error('Controlled element needs an id attribute.');
+    }
+
+    const output = telepath.unpack(this.dataValue);
+    output.render(element, id, ...this.argumentsValue);
+    this.dispatch('ready', { detail: { ...output }, cancelable: false });
+  }
+
+  static afterLoad() {
+    /**
+     * Provide a backwards compatible version of the original window global function.
+     *
+     * @deprecated RemovedInWagtail70
+     */
+    window.initBlockWidget = (id: string) => {
+      const body = document.querySelector(
+        '#' + id + '[data-block]',
+      ) as HTMLElement;
+
+      if (!body) {
+        return;
+      }
+
+      const blockDefData = JSON.parse(body.dataset.data as string);
+      if (window.telepath) {
+        const blockDef = window.telepath.unpack(blockDefData);
+        const blockValue = JSON.parse(body.dataset.value as string);
+        const blockError = JSON.parse(body.dataset.error as string);
+
+        blockDef.render(body, id, blockValue, blockError);
+      }
+    };
+  }
+}
diff --git a/client/src/controllers/index.ts b/client/src/controllers/index.ts
index a5635fbb87..1a19366bb8 100644
--- a/client/src/controllers/index.ts
+++ b/client/src/controllers/index.ts
@@ -3,6 +3,7 @@ import type { Definition } from '@hotwired/stimulus';
 // Order controller imports alphabetically.
 import { ActionController } from './ActionController';
 import { AutosizeController } from './AutosizeController';
+import { BlockController } from './BlockController';
 import { BulkController } from './BulkController';
 import { ClipboardController } from './ClipboardController';
 import { CloneController } from './CloneController';
@@ -34,6 +35,7 @@ export const coreControllerDefinitions: Definition[] = [
   // Keep this list in alphabetical order
   { controllerConstructor: ActionController, identifier: 'w-action' },
   { controllerConstructor: AutosizeController, identifier: 'w-autosize' },
+  { controllerConstructor: BlockController, identifier: 'w-block' },
   { controllerConstructor: BulkController, identifier: 'w-bulk' },
   { controllerConstructor: ClipboardController, identifier: 'w-clipboard' },
   { controllerConstructor: CloneController, identifier: 'w-clone' },