diff --git a/packages/happy-dom/src/fetch/Request.ts b/packages/happy-dom/src/fetch/Request.ts
index 9784f974f..cfe241ca3 100644
--- a/packages/happy-dom/src/fetch/Request.ts
+++ b/packages/happy-dom/src/fetch/Request.ts
@@ -86,6 +86,9 @@ export default class Request implements IRequest {
 		}
 
 		if (contentType) {
+			if (!this.headers.has('Content-Type')) {
+				this.headers.set('Content-Type', contentType);
+			}
 			this._contentType = contentType;
 		} else if (input instanceof Request && input._contentType) {
 			this._contentType = input._contentType;
diff --git a/packages/happy-dom/test/fetch/Request.test.ts b/packages/happy-dom/test/fetch/Request.test.ts
index 46068f12e..fcceb42af 100644
--- a/packages/happy-dom/test/fetch/Request.test.ts
+++ b/packages/happy-dom/test/fetch/Request.test.ts
@@ -244,6 +244,16 @@ describe('Request', () => {
 			expect(request._contentType).toBe('text/plain;charset=UTF-8');
 		});
 
+		it('Supports content type header from Request object.', () => {
+			const request = new Request(new Request(TEST_URL, { method: 'POST', body: 'Hello World' }));
+			expect(request.headers.get('Content-Type')).toBe('text/plain;charset=UTF-8');
+		});
+
+		it('Supports content type header from init object.', () => {
+			const request = new Request(TEST_URL, { method: 'POST', body: 'Hello World' });
+			expect(request.headers.get('Content-Type')).toBe('text/plain;charset=UTF-8');
+		});
+
 		it('Supports redirect from Request object.', () => {
 			const request = new Request(new Request(TEST_URL, { redirect: 'manual' }));
 			expect(request.redirect).toBe('manual');