diff --git a/packages/happy-dom/src/nodes/html-dialog-element/HTMLDialogElement.ts b/packages/happy-dom/src/nodes/html-dialog-element/HTMLDialogElement.ts
index abab3c869..bcf8c4d5a 100644
--- a/packages/happy-dom/src/nodes/html-dialog-element/HTMLDialogElement.ts
+++ b/packages/happy-dom/src/nodes/html-dialog-element/HTMLDialogElement.ts
@@ -49,7 +49,7 @@ export default class HTMLDialogElement extends HTMLElement {
 	 * @param value Return value.
 	 */
 	public set returnValue(value: string) {
-		this[PropertySymbol.returnValue] = value;
+		this[PropertySymbol.returnValue] = String(value);
 	}
 
 	/**
@@ -79,10 +79,10 @@ export default class HTMLDialogElement extends HTMLElement {
 	 *
 	 * @param [returnValue] ReturnValue.
 	 */
-	public close(returnValue = ''): void {
+	public close(returnValue?: string): void {
 		const wasOpen = this.open;
 		this.removeAttribute('open');
-		this.returnValue = returnValue;
+		this.returnValue = returnValue !== undefined ? String(returnValue) : '';
 		if (wasOpen) {
 			this.dispatchEvent(new Event('close'));
 		}
diff --git a/packages/happy-dom/src/nodes/html-form-element/HTMLFormElement.ts b/packages/happy-dom/src/nodes/html-form-element/HTMLFormElement.ts
index de2cd5a27..5ea29f4e1 100644
--- a/packages/happy-dom/src/nodes/html-form-element/HTMLFormElement.ts
+++ b/packages/happy-dom/src/nodes/html-form-element/HTMLFormElement.ts
@@ -17,6 +17,7 @@ import ClassMethodBinder from '../../utilities/ClassMethodBinder.js';
 import Node from '../node/Node.js';
 import Element from '../element/Element.js';
 import EventTarget from '../../event/EventTarget.js';
+import HTMLDialogElement from '../html-dialog-element/HTMLDialogElement.js';
 import ElementEventAttributeUtility from '../element/ElementEventAttributeUtility.js';
 
 /**
@@ -573,6 +574,26 @@ export default class HTMLFormElement extends HTMLElement {
 	 * @param [submitter] Submitter.
 	 */
 	#submit(submitter?: HTMLInputElement | HTMLButtonElement): void {
+		const method = submitter?.formMethod || this.method;
+
+		if (method === 'dialog') {
+			let dialog: HTMLDialogElement | null = null;
+			let parent: Element = this;
+
+			while (parent) {
+				if (parent[PropertySymbol.tagName] === 'DIALOG') {
+					dialog = <HTMLDialogElement>parent;
+					break;
+				}
+				parent = parent.parentElement;
+			}
+
+			if (dialog) {
+				dialog.close(submitter?.value);
+				return;
+			}
+		}
+
 		const action = submitter?.hasAttribute('formaction')
 			? submitter?.formAction || this.action
 			: this.action;
@@ -589,7 +610,6 @@ export default class HTMLFormElement extends HTMLElement {
 			return;
 		}
 
-		const method = submitter?.formMethod || this.method;
 		const formData = new this[PropertySymbol.window].FormData(this);
 		let targetFrame: IBrowserFrame;
 
diff --git a/packages/happy-dom/test/nodes/html-dialog-element/HTMLDialogElement.test.ts b/packages/happy-dom/test/nodes/html-dialog-element/HTMLDialogElement.test.ts
index ca2304af7..f6a847dfe 100644
--- a/packages/happy-dom/test/nodes/html-dialog-element/HTMLDialogElement.test.ts
+++ b/packages/happy-dom/test/nodes/html-dialog-element/HTMLDialogElement.test.ts
@@ -3,6 +3,7 @@ import HTMLDialogElement from '../../../src/nodes/html-dialog-element/HTMLDialog
 import Window from '../../../src/window/Window.js';
 import Document from '../../../src/nodes/document/Document.js';
 import { beforeEach, describe, it, expect } from 'vitest';
+import KeyboardEvent from '../../../src/event/events/KeyboardEvent.js';
 
 describe('HTMLDialogElement', () => {
 	let window: Window;
@@ -83,6 +84,8 @@ describe('HTMLDialogElement', () => {
 		it('Should be possible to set manually', () => {
 			element.returnValue = 'foo';
 			expect(element.returnValue).toBe('foo');
+			element.returnValue = <string>(<unknown>undefined);
+			expect(element.returnValue).toBe('undefined');
 		});
 	});
 
@@ -105,6 +108,12 @@ describe('HTMLDialogElement', () => {
 			element.show();
 			element.close('foo');
 			expect(element.returnValue).toBe('foo');
+			element.show();
+			element.close(undefined);
+			expect(element.returnValue).toBe('');
+			element.show();
+			element.close(<string>(<unknown>null));
+			expect(element.returnValue).toBe('null');
 		});
 
 		it('Should be possible to close the modal dialog with a return value', () => {
@@ -122,7 +131,7 @@ describe('HTMLDialogElement', () => {
 			expect((<Event>(<unknown>dispatched)).bubbles).toBe(false);
 		});
 
-		it('Should only dispatch a close event when dialog wasnt already closed', () => {
+		it("Should only dispatch a close event when dialog wasn't already closed", () => {
 			let dispatched: Event | null = null;
 			element.addEventListener('close', (event: Event) => (dispatched = event));
 			element.close();
diff --git a/packages/happy-dom/test/nodes/html-form-element/HTMLFormElement.test.ts b/packages/happy-dom/test/nodes/html-form-element/HTMLFormElement.test.ts
index 8b6f9986f..b684837af 100644
--- a/packages/happy-dom/test/nodes/html-form-element/HTMLFormElement.test.ts
+++ b/packages/happy-dom/test/nodes/html-form-element/HTMLFormElement.test.ts
@@ -930,6 +930,46 @@ describe('HTMLFormElement', () => {
 
 			expect(page.mainFrame.url).toBe('about:blank#blocked');
 		});
+
+		it('Supports form method "dialog"', () => {
+			const container = document.body;
+			container.innerHTML = `<dialog>
+										<form method="dialog">
+											<input name="test123" value="">
+											<button value="buttonValue">Close</button>
+										</form>
+									</dialog>`;
+			const dialog = container.querySelector('dialog')!;
+			const form = dialog.querySelector('form')!;
+			const button = dialog.querySelector('button')!;
+			const input = dialog.querySelector('input')!;
+
+			input.value = 'test';
+
+			expect(dialog.returnValue).toBe('');
+			expect(dialog.open).toBe(false);
+
+			dialog.showModal();
+
+			expect(dialog.open).toBe(true);
+
+			let submitEvent: SubmitEvent | null = null;
+			dialog.addEventListener('submit', (event) => (submitEvent = <SubmitEvent>event));
+			button.click();
+			expect(dialog.open).toBe(false);
+			expect((<SubmitEvent>(<unknown>submitEvent)).submitter).toBe(button);
+			expect((<SubmitEvent>(<unknown>submitEvent)).target).toBe(form);
+			expect(dialog.returnValue).toBe('buttonValue');
+
+			dialog.showModal();
+			expect(dialog.open).toBe(true);
+
+			form.submit();
+			expect(dialog.open).toBe(false);
+			expect(dialog.returnValue).toBe('');
+
+			expect(input.value).toBe('test');
+		});
 	});
 
 	describe('requestSubmit()', () => {