diff --git a/source/core/index.ts b/source/core/index.ts
index 7973b1a33..4b11fdec2 100644
--- a/source/core/index.ts
+++ b/source/core/index.ts
@@ -1608,14 +1608,14 @@ export default class Request extends Duplex implements RequestEvents<Request> {
 		options.password = options.password ?? '';
 
 		// `options.prefixUrl` & `options.url`
-		if (options.prefixUrl) {
+		if (is.undefined(options.prefixUrl)) {
+			options.prefixUrl = defaults?.prefixUrl ?? '';
+		} else {
 			options.prefixUrl = options.prefixUrl.toString();
 
 			if (options.prefixUrl !== '' && !options.prefixUrl.endsWith('/')) {
 				options.prefixUrl += '/';
 			}
-		} else {
-			options.prefixUrl = '';
 		}
 
 		if (is.string(options.url)) {
diff --git a/test/arguments.ts b/test/arguments.ts
index 17ff04450..5088b898a 100644
--- a/test/arguments.ts
+++ b/test/arguments.ts
@@ -97,12 +97,12 @@ test('methods are normalized', withServer, async (t, server, got) => {
 	await instance('test', {method: 'post'});
 });
 
-test.failing('throws an error when legacy URL is passed', withServer, async (t, server, got) => {
+test.failing('throws an error when legacy URL is passed', withServer, async (t, server) => {
 	server.get('/test', echoUrl);
 
 	await t.throwsAsync(
 		// @ts-expect-error Error tests
-		got(parse(`${server.url}/test`), {prefixUrl: ''}),
+		got(parse(`${server.url}/test`)),
 		{message: 'The legacy `url.Url` has been deprecated. Use `URL` instead.'}
 	);
 
@@ -110,22 +110,20 @@ test.failing('throws an error when legacy URL is passed', withServer, async (t,
 		got({
 			protocol: 'http:',
 			hostname: 'localhost',
-			port: server.port,
-			prefixUrl: ''
+			port: server.port
 		}),
 		{message: 'The legacy `url.Url` has been deprecated. Use `URL` instead.'}
 	);
 });
 
-test('accepts legacy URL options', withServer, async (t, server, got) => {
+test('accepts legacy URL options', withServer, async (t, server) => {
 	server.get('/test', echoUrl);
 
 	const {body: secondBody} = await got({
 		protocol: 'http:',
 		hostname: 'localhost',
 		port: server.port,
-		pathname: '/test',
-		prefixUrl: ''
+		pathname: '/test'
 	});
 
 	t.is(secondBody, '/test');
diff --git a/test/cache.ts b/test/cache.ts
index b31eea088..e9532211e 100644
--- a/test/cache.ts
+++ b/test/cache.ts
@@ -143,10 +143,9 @@ test('doesn\'t cache response when received HTTP error', withServer, async (t, s
 	t.is(body, 'ok');
 });
 
-test('DNS cache works', withServer, async (t, _server, got) => {
+test('DNS cache works', async t => {
 	const instance = got.extend({
-		dnsCache: true,
-		prefixUrl: ''
+		dnsCache: true
 	});
 
 	await t.notThrowsAsync(instance('https://example.com'));
@@ -155,9 +154,9 @@ test('DNS cache works', withServer, async (t, _server, got) => {
 	t.is(instance.defaults.options.dnsCache!._cache.size, 1);
 });
 
-test('DNS cache works - CacheableLookup instance', withServer, async (t, _server, got) => {
+test('DNS cache works - CacheableLookup instance', async t => {
 	const cache = new CacheableLookup();
-	await t.notThrowsAsync(got('https://example.com', {dnsCache: cache, prefixUrl: ''}));
+	await t.notThrowsAsync(got('https://example.com', {dnsCache: cache}));
 
 	t.is((cache as any)._cache.size, 1);
 });
diff --git a/test/merge-instances.ts b/test/merge-instances.ts
index 8798fd1df..f32141e39 100644
--- a/test/merge-instances.ts
+++ b/test/merge-instances.ts
@@ -160,3 +160,18 @@ test('accepts options for promise API', t => {
 
 	t.pass();
 });
+
+test('merging `prefixUrl`', t => {
+	const prefixUrl = 'http://example.com/';
+
+	const instanceA = got.extend({headers: {unicorn: 'rainbow'}});
+	const instanceB = got.extend({prefixUrl});
+	const mergedAonB = instanceB.extend(instanceA);
+	const mergedBonA = instanceA.extend(instanceB);
+
+	t.is(mergedAonB.defaults.options.prefixUrl, '');
+	t.is(mergedBonA.defaults.options.prefixUrl, prefixUrl);
+
+	t.is(instanceB.extend({}).defaults.options.prefixUrl, prefixUrl);
+	t.is(instanceB.extend({prefixUrl: undefined}).defaults.options.prefixUrl, prefixUrl);
+});
diff --git a/test/stream.ts b/test/stream.ts
index 508098e5b..61e5f2528 100644
--- a/test/stream.ts
+++ b/test/stream.ts
@@ -154,8 +154,8 @@ test('has error event', withServer, async (t, server, got) => {
 	});
 });
 
-test('has error event #2', withServer, async (t, _server, got) => {
-	const stream = got.stream('http://doesntexist', {prefixUrl: ''});
+test('has error event #2', async t => {
+	const stream = got.stream('http://doesntexist');
 	await t.throwsAsync(pEvent(stream, 'response'), {code: 'ENOTFOUND'});
 });
 
diff --git a/test/timeout.ts b/test/timeout.ts
index 1d07bd9e7..dc09328b0 100644
--- a/test/timeout.ts
+++ b/test/timeout.ts
@@ -249,11 +249,10 @@ test.serial('connect timeout', withServerAndFakeTimers, async (t, _server, got,
 	);
 });
 
-test.serial('connect timeout (ip address)', withServerAndFakeTimers, async (t, _server, got, clock) => {
+test.serial('connect timeout (ip address)', withServerAndFakeTimers, async (t, _server, _got, clock) => {
 	await t.throwsAsync(
 		got({
 			url: 'http://127.0.0.1',
-			prefixUrl: '',
 			createConnection: options => {
 				const socket = new net.Socket(options as Record<string, unknown> as net.SocketConstructorOpts);
 				// @ts-expect-error We know that it is readonly, but we have to test it
@@ -338,12 +337,11 @@ test.serial('lookup timeout', withServerAndFakeTimers, async (t, server, got, cl
 	);
 });
 
-test.serial('lookup timeout no error (ip address)', withServerAndFakeTimers, async (t, server, got, clock) => {
+test.serial('lookup timeout no error (ip address)', withServerAndFakeTimers, async (t, server, _got, clock) => {
 	server.get('/', defaultHandler(clock));
 
 	await t.notThrowsAsync(got({
 		url: `http://127.0.0.1:${server.port}`,
-		prefixUrl: '',
 		timeout: {lookup: 1},
 		retry: 0
 	}));