diff --git a/packages/happy-dom/src/fetch/Request.ts b/packages/happy-dom/src/fetch/Request.ts
index b5ab746d..0852a1d3 100644
--- a/packages/happy-dom/src/fetch/Request.ts
+++ b/packages/happy-dom/src/fetch/Request.ts
@@ -304,29 +304,50 @@ export default class Request implements Request {
 	 * @returns FormData.
 	 */
 	public async formData(): Promise<FormData> {
-		if (this.bodyUsed) {
-			throw new DOMException(
-				`Body has already been used for "${this.url}".`,
-				DOMExceptionNameEnum.invalidStateError
-			);
-		}
+		const contentType = this[PropertySymbol.contentType];
+		const asyncTaskManager = this.#asyncTaskManager;
 
-		(<boolean>this.bodyUsed) = true;
+		if (/multipart/i.test(contentType)) {
+			if (this.bodyUsed) {
+				throw new DOMException(
+					`Body has already been used for "${this.url}".`,
+					DOMExceptionNameEnum.invalidStateError
+				);
+			}
 
-		const taskID = this.#asyncTaskManager.startTask(() => this.signal[PropertySymbol.abort]());
-		let formData: FormData;
+			(<boolean>this.bodyUsed) = true;
 
-		try {
-			const type = this[PropertySymbol.contentType];
-			formData = (await MultipartFormDataParser.streamToFormData(this.body, type)).formData;
-		} catch (error) {
-			this.#asyncTaskManager.endTask(taskID);
-			throw error;
+			const taskID = asyncTaskManager.startTask(() => this.signal[PropertySymbol.abort]());
+			let formData: FormData;
+
+			try {
+				const result = await MultipartFormDataParser.streamToFormData(this.body, contentType);
+				formData = result.formData;
+			} catch (error) {
+				asyncTaskManager.endTask(taskID);
+				throw error;
+			}
+
+			asyncTaskManager.endTask(taskID);
+
+			return formData;
 		}
 
-		this.#asyncTaskManager.endTask(taskID);
+		if (contentType?.startsWith('application/x-www-form-urlencoded')) {
+			const parameters = new URLSearchParams(await this.text());
+			const formData = new FormData();
+
+			for (const [key, value] of parameters) {
+				formData.append(key, value);
+			}
 
-		return formData;
+			return formData;
+		}
+
+		throw new DOMException(
+			`Failed to construct FormData object: The "content-type" header is neither "application/x-www-form-urlencoded" nor "multipart/form-data".`,
+			DOMExceptionNameEnum.invalidStateError
+		);
 	}
 
 	/**
diff --git a/packages/happy-dom/src/fetch/Response.ts b/packages/happy-dom/src/fetch/Response.ts
index cfdc9974..ee915208 100644
--- a/packages/happy-dom/src/fetch/Response.ts
+++ b/packages/happy-dom/src/fetch/Response.ts
@@ -225,37 +225,52 @@ export default class Response implements Response {
 	 */
 	public async formData(): Promise<FormData> {
 		const contentType = this.headers.get('Content-Type');
+		const asyncTaskManager = this.#browserFrame[PropertySymbol.asyncTaskManager];
+
+		if (/multipart/i.test(contentType)) {
+			if (this.bodyUsed) {
+				throw new DOMException(
+					`Body has already been used for "${this.url}".`,
+					DOMExceptionNameEnum.invalidStateError
+				);
+			}
 
-		if (contentType?.startsWith('application/x-www-form-urlencoded')) {
-			const formData = new FormData();
-			const text = await this.text();
-			const parameters = new URLSearchParams(text);
+			(<boolean>this.bodyUsed) = true;
 
-			for (const [name, value] of parameters) {
-				formData.append(name, value);
+			const taskID = asyncTaskManager.startTask();
+			let formData: FormData;
+			let buffer: Buffer;
+
+			try {
+				const result = await MultipartFormDataParser.streamToFormData(this.body, contentType);
+				formData = result.formData;
+				buffer = result.buffer;
+			} catch (error) {
+				asyncTaskManager.endTask(taskID);
+				throw error;
 			}
 
+			this.#storeBodyInCache(buffer);
+			asyncTaskManager.endTask(taskID);
+
 			return formData;
 		}
 
-		const taskID = this.#browserFrame[PropertySymbol.asyncTaskManager].startTask();
-		let formData: FormData;
-		let buffer: Buffer;
-
-		try {
-			const result = await MultipartFormDataParser.streamToFormData(this.body, contentType);
-			formData = result.formData;
-			buffer = result.buffer;
-		} catch (error) {
-			this.#browserFrame[PropertySymbol.asyncTaskManager].endTask(taskID);
-			throw error;
-		}
+		if (contentType?.startsWith('application/x-www-form-urlencoded')) {
+			const parameters = new URLSearchParams(await this.text());
+			const formData = new FormData();
 
-		this.#storeBodyInCache(buffer);
+			for (const [key, value] of parameters) {
+				formData.append(key, value);
+			}
 
-		this.#browserFrame[PropertySymbol.asyncTaskManager].endTask(taskID);
+			return formData;
+		}
 
-		return formData;
+		throw new DOMException(
+			`Failed to build FormData object: The "content-type" header is neither "application/x-www-form-urlencoded" nor "multipart/form-data".`,
+			DOMExceptionNameEnum.invalidStateError
+		);
 	}
 
 	/**
diff --git a/packages/happy-dom/test/fetch/Request.test.ts b/packages/happy-dom/test/fetch/Request.test.ts
index 8d598464..e7378cfb 100644
--- a/packages/happy-dom/test/fetch/Request.test.ts
+++ b/packages/happy-dom/test/fetch/Request.test.ts
@@ -15,6 +15,9 @@ import MultipartFormDataParser from '../../src/fetch/multipart/MultipartFormData
 import { beforeEach, describe, it, expect, vi, afterEach } from 'vitest';
 import { ReadableStream } from 'stream/web';
 import * as PropertySymbol from '../../src/PropertySymbol.js';
+import File from '../../src/file/File.js';
+import Path from 'path';
+import FS from 'fs';
 
 const TEST_URL = 'https://example.com/';
 
@@ -666,13 +669,106 @@ describe('Request', () => {
 	});
 
 	describe('formData()', () => {
-		it('Returns FormData', async () => {
+		it('Returns FormData for FormData object (multipart)', async () => {
 			const formData = new FormData();
 			formData.append('some', 'test');
 			const request = new window.Request(TEST_URL, { method: 'POST', body: formData });
-			const requestFormData = await request.formData();
+			const formDataResponse = await request.formData();
 
-			expect(requestFormData).toEqual(formData);
+			expect(formDataResponse).toEqual(formData);
+		});
+
+		it('Returns FormData for URLSearchParams object (application/x-www-form-urlencoded)', async () => {
+			const urlSearchParams = new URLSearchParams();
+			urlSearchParams.append('some', 'test');
+			const request = new window.Request(TEST_URL, { method: 'POST', body: urlSearchParams });
+			const formDataResponse = await request.formData();
+
+			expect(formDataResponse instanceof FormData).toBe(true);
+			expect(formDataResponse.get('some')).toBe('test');
+		});
+
+		it('Returns FormData for "application/x-www-form-urlencoded" content.', async () => {
+			const urlSearchParams = new URLSearchParams();
+
+			urlSearchParams.set('key1', 'value1');
+			urlSearchParams.set('key2', 'value2');
+			urlSearchParams.set('key3', 'value3');
+
+			const request = new window.Request(TEST_URL, { method: 'POST', body: urlSearchParams });
+			const formDataResponse = await request.formData();
+			let size = 0;
+
+			for (const _entry of formDataResponse) {
+				size++;
+			}
+
+			expect(formDataResponse.get('key1')).toBe('value1');
+			expect(formDataResponse.get('key2')).toBe('value2');
+			expect(formDataResponse.get('key3')).toBe('value3');
+			expect(size).toBe(3);
+		});
+
+		it('Returns FormData for multipart text fields.', async () => {
+			const formData = new FormData();
+
+			vi.spyOn(Math, 'random').mockImplementation(() => 0.8);
+
+			formData.set('key1', 'value1');
+			formData.set('key2', 'value2');
+			formData.set('key3', 'value3');
+
+			const request = new window.Request(TEST_URL, { method: 'POST', body: formData });
+			const formDataResponse = await request.formData();
+			let size = 0;
+
+			for (const _entry of formDataResponse) {
+				size++;
+			}
+
+			expect(formDataResponse.get('key1')).toBe('value1');
+			expect(formDataResponse.get('key2')).toBe('value2');
+			expect(formDataResponse.get('key3')).toBe('value3');
+			expect(size).toBe(3);
+		});
+
+		it('Returns FormData for multipart files.', async () => {
+			const formData = new FormData();
+			const imageBuffer = await FS.promises.readFile(
+				Path.join(__dirname, 'data', 'test-image.jpg')
+			);
+
+			vi.spyOn(Math, 'random').mockImplementation(() => 0.8);
+
+			formData.set('key1', 'value1');
+			formData.set('file1', new File([imageBuffer], 'test-image-1.jpg', { type: 'image/jpeg' }));
+			formData.set('key2', 'value2');
+			formData.set('file2', new File([imageBuffer], 'test-image-2.jpg', { type: 'image/jpeg' }));
+
+			const request = new window.Request(TEST_URL, { method: 'POST', body: formData });
+			const formDataResponse = await request.formData();
+			let size = 0;
+
+			for (const _entry of formDataResponse) {
+				size++;
+			}
+
+			expect(formDataResponse.get('key1')).toBe('value1');
+			expect(formDataResponse.get('key2')).toBe('value2');
+			expect(size).toBe(4);
+
+			const file1 = <File>formDataResponse.get('file1');
+			const file2 = <File>formDataResponse.get('file2');
+
+			expect(file1.name).toBe('test-image-1.jpg');
+			expect(file1.type).toBe('image/jpeg');
+			expect(file1.size).toBe(imageBuffer.length);
+			expect(await file1.arrayBuffer()).toEqual(imageBuffer.buffer);
+
+			expect(file2.name).toBe('test-image-2.jpg');
+			expect(file2.type).toBe('image/jpeg');
+			expect(file2.size).toBe(imageBuffer.length);
+			expect(await file2.arrayBuffer()).toEqual(imageBuffer.buffer);
 		});
 
 		it('Supports window.happyDOM?.waitUntilComplete().', async () => {
diff --git a/packages/happy-dom/test/fetch/Response.test.ts b/packages/happy-dom/test/fetch/Response.test.ts
index 10a46d16..9c8e523a 100644
--- a/packages/happy-dom/test/fetch/Response.test.ts
+++ b/packages/happy-dom/test/fetch/Response.test.ts
@@ -321,6 +321,25 @@ describe('Response', () => {
 	});
 
 	describe('formData()', () => {
+		it('Returns FormData for FormData object (multipart)', async () => {
+			const formData = new FormData();
+			formData.append('some', 'test');
+			const response = new window.Response(formData);
+			const formDataResponse = await response.formData();
+
+			expect(formDataResponse).toEqual(formData);
+		});
+
+		it('Returns FormData for URLSearchParams object (application/x-www-form-urlencoded)', async () => {
+			const urlSearchParams = new URLSearchParams();
+			urlSearchParams.append('some', 'test');
+			const response = new window.Response(urlSearchParams);
+			const formDataResponse = await response.formData();
+
+			expect(formDataResponse instanceof FormData).toBe(true);
+			expect(formDataResponse.get('some')).toBe('test');
+		});
+
 		it('Returns FormData for "application/x-www-form-urlencoded" content.', async () => {
 			const urlSearchParams = new URLSearchParams();