diff --git a/packages/happy-dom/src/PropertySymbol.ts b/packages/happy-dom/src/PropertySymbol.ts
index c13aebf1..e3b7fc15 100644
--- a/packages/happy-dom/src/PropertySymbol.ts
+++ b/packages/happy-dom/src/PropertySymbol.ts
@@ -158,3 +158,4 @@ export const navigator = Symbol('navigator');
 export const screen = Symbol('screen');
 export const sessionStorage = Symbol('sessionStorage');
 export const localStorage = Symbol('localStorage');
+export const sandbox = Symbol('sandbox');
diff --git a/packages/happy-dom/src/nodes/html-iframe-element/HTMLIFrameElement.ts b/packages/happy-dom/src/nodes/html-iframe-element/HTMLIFrameElement.ts
index 1cffbdb1..d43fb63e 100644
--- a/packages/happy-dom/src/nodes/html-iframe-element/HTMLIFrameElement.ts
+++ b/packages/happy-dom/src/nodes/html-iframe-element/HTMLIFrameElement.ts
@@ -9,6 +9,7 @@ import HTMLIFrameElementNamedNodeMap from './HTMLIFrameElementNamedNodeMap.js';
 import CrossOriginBrowserWindow from '../../window/CrossOriginBrowserWindow.js';
 import IBrowserFrame from '../../browser/types/IBrowserFrame.js';
 import HTMLIFrameElementPageLoader from './HTMLIFrameElementPageLoader.js';
+import DOMTokenList from '../../dom-token-list/DOMTokenList.js';
 
 /**
  * HTML Iframe Element.
@@ -23,7 +24,7 @@ export default class HTMLIFrameElement extends HTMLElement {
 
 	// Internal properties
 	public override [PropertySymbol.attributes]: NamedNodeMap;
-
+	public [PropertySymbol.sandbox]: DOMTokenList = null;
 	// Private properties
 	#contentWindowContainer: { window: BrowserWindow | CrossOriginBrowserWindow | null } = {
 		window: null
@@ -140,14 +141,15 @@ export default class HTMLIFrameElement extends HTMLElement {
 	 *
 	 * @returns Sandbox.
 	 */
-	public get sandbox(): string {
-		return this.getAttribute('sandbox') || '';
+	public get sandbox(): DOMTokenList {
+		if (!this[PropertySymbol.sandbox]) {
+			this[PropertySymbol.sandbox] = new DOMTokenList(this, 'sandbox');
+		}
+		return <DOMTokenList>this[PropertySymbol.sandbox];
 	}
 
 	/**
 	 * Sets sandbox.
-	 *
-	 * @param sandbox Sandbox.
 	 */
 	public set sandbox(sandbox: string) {
 		this.setAttribute('sandbox', sandbox);
@@ -163,7 +165,7 @@ export default class HTMLIFrameElement extends HTMLElement {
 	}
 
 	/**
-	 * Sets sandbox.
+	 * Sets srcdoc.
 	 *
 	 * @param srcdoc Srcdoc.
 	 */
diff --git a/packages/happy-dom/src/nodes/html-iframe-element/HTMLIFrameElementNamedNodeMap.ts b/packages/happy-dom/src/nodes/html-iframe-element/HTMLIFrameElementNamedNodeMap.ts
index 3a6329e4..28a8be90 100644
--- a/packages/happy-dom/src/nodes/html-iframe-element/HTMLIFrameElementNamedNodeMap.ts
+++ b/packages/happy-dom/src/nodes/html-iframe-element/HTMLIFrameElementNamedNodeMap.ts
@@ -3,6 +3,23 @@ import Element from '../element/Element.js';
 import HTMLElementNamedNodeMap from '../html-element/HTMLElementNamedNodeMap.js';
 import HTMLIFrameElementPageLoader from './HTMLIFrameElementPageLoader.js';
 import * as PropertySymbol from '../../PropertySymbol.js';
+import DOMTokenList from '../../dom-token-list/DOMTokenList.js';
+
+const SANDBOX_FLAGS = [
+	'allow-downloads',
+	'allow-forms',
+	'allow-modals',
+	'allow-orientation-lock',
+	'allow-pointer-lock',
+	'allow-popups',
+	'allow-popups-to-escape-sandbox',
+	'allow-presentation',
+	'allow-same-origin',
+	'allow-scripts',
+	'allow-top-navigation',
+	'allow-top-navigation-by-user-activation',
+	'allow-top-navigation-to-custom-protocols'
+];
 
 /**
  * Named Node Map.
@@ -37,6 +54,49 @@ export default class HTMLIFrameElementNamedNodeMap extends HTMLElementNamedNodeM
 			this.#pageLoader.loadPage();
 		}
 
+		if (item[PropertySymbol.name] === 'sandbox') {
+			if (!this[PropertySymbol.ownerElement][PropertySymbol.sandbox]) {
+				this[PropertySymbol.ownerElement][PropertySymbol.sandbox] = new DOMTokenList(
+					this[PropertySymbol.ownerElement],
+					'sandbox'
+				);
+			} else {
+				this[PropertySymbol.ownerElement][PropertySymbol.sandbox][PropertySymbol.updateIndices]();
+			}
+
+			this.#validateSandboxFlags();
+		}
+
 		return replacedAttribute || null;
 	}
+
+	/**
+	 *
+	 * @param tokens
+	 * @param vconsole
+	 */
+	#validateSandboxFlags(): void {
+		const window =
+			this[PropertySymbol.ownerElement][PropertySymbol.ownerDocument][PropertySymbol.ownerWindow];
+		const values = this[PropertySymbol.ownerElement][PropertySymbol.sandbox].values();
+		const invalidFlags: string[] = [];
+
+		for (const token of values) {
+			if (!SANDBOX_FLAGS.includes(token)) {
+				invalidFlags.push(token);
+			}
+		}
+
+		if (invalidFlags.length === 1) {
+			window.console.error(
+				`Error while parsing the 'sandbox' attribute: '${invalidFlags[0]}' is an invalid sandbox flag.`
+			);
+		} else if (invalidFlags.length > 1) {
+			window.console.error(
+				`Error while parsing the 'sandbox' attribute: '${invalidFlags.join(
+					`', '`
+				)}' are invalid sandbox flags.`
+			);
+		}
+	}
 }
diff --git a/packages/happy-dom/test/dom-token-list/DOMTokenList.test.ts b/packages/happy-dom/test/dom-token-list/DOMTokenList.test.ts
index ddc8c717..5fa2f00a 100644
--- a/packages/happy-dom/test/dom-token-list/DOMTokenList.test.ts
+++ b/packages/happy-dom/test/dom-token-list/DOMTokenList.test.ts
@@ -85,6 +85,7 @@ describe('DOMTokenList', () => {
 		it('Sets the attribute value.', () => {
 			classList.value = 'class1 class2 class3';
 			expect(element.className).toBe('class1 class2 class3');
+			expect(classList[2]).toBe('class3');
 		});
 	});
 
diff --git a/packages/happy-dom/test/nodes/html-iframe-element/HTMLIFrameElement.test.ts b/packages/happy-dom/test/nodes/html-iframe-element/HTMLIFrameElement.test.ts
index fdcad0c6..74d75024 100644
--- a/packages/happy-dom/test/nodes/html-iframe-element/HTMLIFrameElement.test.ts
+++ b/packages/happy-dom/test/nodes/html-iframe-element/HTMLIFrameElement.test.ts
@@ -1,6 +1,5 @@
 import Window from '../../../src/window/Window.js';
 import BrowserWindow from '../../../src/window/BrowserWindow.js';
-import Window from '../../../src/window/Window.js';
 import Document from '../../../src/nodes/document/Document.js';
 import HTMLIFrameElement from '../../../src/nodes/html-iframe-element/HTMLIFrameElement.js';
 import Response from '../../../src/fetch/Response.js';
@@ -13,6 +12,7 @@ import { beforeEach, describe, it, expect, vi, afterEach } from 'vitest';
 import IRequestInfo from '../../../src/fetch/types/IRequestInfo.js';
 import Headers from '../../../src/fetch/Headers.js';
 import Browser from '../../../src/browser/Browser.js';
+import DOMTokenList from '../../../src/dom-token-list/DOMTokenList.js';
 
 describe('HTMLIFrameElement', () => {
 	let window: Window;
@@ -35,7 +35,7 @@ describe('HTMLIFrameElement', () => {
 		});
 	});
 
-	for (const property of ['src', 'allow', 'height', 'width', 'name', 'sandbox', 'srcdoc']) {
+	for (const property of ['src', 'allow', 'height', 'width', 'name', 'srcdoc']) {
 		describe(`get ${property}()`, () => {
 			it(`Returns the "${property}" attribute.`, () => {
 				element.setAttribute(property, 'value');
@@ -51,6 +51,81 @@ describe('HTMLIFrameElement', () => {
 		});
 	}
 
+	describe('get sandbox()', () => {
+		it('Returns DOMTokenList', () => {
+			expect(element.sandbox).toBeInstanceOf(DOMTokenList);
+			element.sandbox.add('allow-forms', 'allow-scripts');
+			expect(element.sandbox.toString()).toBe('allow-forms allow-scripts');
+		});
+
+		it('Returns values from attribute', () => {
+			element.setAttribute('sandbox', 'allow-forms allow-scripts');
+			expect(element.sandbox.toString()).toBe('allow-forms allow-scripts');
+		});
+	});
+
+	describe('set sandbox()', () => {
+		it('Sets attribute', () => {
+			element.sandbox = 'allow-forms allow-scripts';
+			expect(element.getAttribute('sandbox')).toBe('allow-forms allow-scripts');
+
+			element.sandbox =
+				'allow-downloads allow-forms allow-modals allow-orientation-lock allow-pointer-lock allow-popups allow-popups-to-escape-sandbox allow-presentation allow-same-origin allow-scripts allow-top-navigation allow-top-navigation-by-user-activation allow-top-navigation-to-custom-protocols';
+			expect(element.sandbox.toString()).toBe(
+				'allow-downloads allow-forms allow-modals allow-orientation-lock allow-pointer-lock allow-popups allow-popups-to-escape-sandbox allow-presentation allow-same-origin allow-scripts allow-top-navigation allow-top-navigation-by-user-activation allow-top-navigation-to-custom-protocols'
+			);
+		});
+
+		it('Updates the DOMTokenList indicies when setting the sandbox attribute', () => {
+			element.sandbox = 'allow-forms allow-scripts';
+			expect(element.sandbox.length).toBe(2);
+			expect(element.sandbox[0]).toBe('allow-forms');
+			expect(element.sandbox[1]).toBe('allow-scripts');
+
+			element.sandbox = 'allow-scripts allow-forms';
+			expect(element.sandbox.length).toBe(2);
+			expect(element.sandbox[0]).toBe('allow-scripts');
+			expect(element.sandbox[1]).toBe('allow-forms');
+
+			element.sandbox = 'allow-forms';
+			expect(element.sandbox.length).toBe(1);
+			expect(element.sandbox[0]).toBe('allow-forms');
+			expect(element.sandbox[1]).toBe(undefined);
+
+			element.sandbox = '';
+
+			expect(element.sandbox.length).toBe(0);
+			expect(element.sandbox[0]).toBe(undefined);
+
+			element.sandbox = 'allow-forms allow-scripts allow-forms';
+			expect(element.sandbox.length).toBe(2);
+			expect(element.sandbox[0]).toBe('allow-forms');
+			expect(element.sandbox[1]).toBe('allow-scripts');
+
+			element.sandbox = 'allow-forms allow-scripts allow-modals';
+			expect(element.sandbox.length).toBe(3);
+			expect(element.sandbox[0]).toBe('allow-forms');
+			expect(element.sandbox[1]).toBe('allow-scripts');
+			expect(element.sandbox[2]).toBe('allow-modals');
+		});
+
+		it('Console error occurs when add an invalid sandbox flag', () => {
+			element.sandbox = 'invalid';
+			expect(window.happyDOM.virtualConsolePrinter.readAsString()).toBe(
+				`Error while parsing the 'sandbox' attribute: 'invalid' is an invalid sandbox flag.\n`
+			);
+			expect(element.sandbox.toString()).toBe('invalid');
+			expect(element.getAttribute('sandbox')).toBe('invalid');
+
+			element.setAttribute('sandbox', 'first-invalid second-invalid');
+			expect(window.happyDOM.virtualConsolePrinter.readAsString()).toBe(
+				`Error while parsing the 'sandbox' attribute: 'first-invalid', 'second-invalid' are invalid sandbox flags.\n`
+			);
+			expect(element.sandbox.toString()).toBe('first-invalid second-invalid');
+			expect(element.getAttribute('sandbox')).toBe('first-invalid second-invalid');
+		});
+	});
+
 	describe('get contentWindow()', () => {
 		it('Returns content window for "about:blank".', () => {
 			element.src = 'about:blank';