diff --git a/src/index.ts b/src/index.ts
index 99bab4e..ecae9c0 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -891,7 +891,8 @@ function createProxyAgent(
 		uri: proxyURL,
 		allowH2,
 		requestTls: requestCA ? { allowH2, ca: requestCA } : { allowH2 },
-		proxyTls: proxyCA ? { allowH2, ca: proxyCA } : { allowH2 },
+		// Supplying proxyTls also enables TLS to SOCKS proxies in undici.
+		proxyTls: new URL(proxyURL).protocol === 'https:' ? { allowH2, ca: proxyCA } : undefined,
 		clientFactory: (origin: URL, opts: object): undici.Dispatcher => (new undici.Pool(origin, opts) as any).compose((dispatch: undici.Dispatcher['dispatch']) => {
 			class ProxyAuthHandler extends undici.DecoratorHandler {
 				private connectResponseHeaders?: IncomingHttpHeaders;
diff --git a/tests/src/socksProxyFetch.test.ts b/tests/src/socksProxyFetch.test.ts
new file mode 100644
index 0000000..36dc24f
--- /dev/null
+++ b/tests/src/socksProxyFetch.test.ts
@@ -0,0 +1,91 @@
+import * as assert from 'assert';
+import { once } from 'events';
+import * as net from 'net';
+import { createFetchPatch, createProxyResolver, LogLevel, ProxyAgentParams, resetCaches } from '../../src';
+
+function createParams(proxyURL: string, addCertificates: boolean): ProxyAgentParams {
+	const noop = () => { };
+	return {
+		resolveProxy: async () => undefined,
+		getProxyURL: () => proxyURL,
+		getProxySupport: () => 'override',
+		isAdditionalFetchSupportEnabled: () => true,
+		isWebSocketPatchEnabled: () => true,
+		addCertificatesV1: () => addCertificates,
+		addCertificatesV2: () => false,
+		loadSystemCertificatesFromNode: () => false,
+		loadAdditionalCertificates: async () => [],
+		log: { trace: noop, debug: noop, info: noop, warn: noop, error: noop },
+		getLogLevel: () => LogLevel.Off,
+		proxyResolveTelemetry: noop,
+		isUseHostProxyEnabled: () => true,
+		env: {},
+	};
+}
+
+async function readBytes(socket: net.Socket, length: number): Promise<Buffer> {
+	let data: Buffer | null;
+	while ((data = socket.read(length)) === null) {
+		if (socket.destroyed || socket.readableEnded) {
+			throw new Error('SOCKS client disconnected');
+		}
+		await once(socket, 'readable');
+	}
+	return data;
+}
+
+async function serveSocksRequest(socket: net.Socket): Promise<void> {
+	// A plain SOCKS proxy must receive a SOCKS greeting, not a TLS ClientHello.
+	assert.deepStrictEqual(await readBytes(socket, 2), Buffer.from([5, 1]));
+	assert.deepStrictEqual(await readBytes(socket, 1), Buffer.from([0]));
+	socket.write(Buffer.from([5, 0]));
+
+	assert.deepStrictEqual(await readBytes(socket, 4), Buffer.from([5, 1, 0, 3]));
+	const hostLength = (await readBytes(socket, 1))[0];
+	assert.strictEqual((await readBytes(socket, hostLength)).toString(), 'example.test');
+	assert.strictEqual((await readBytes(socket, 2)).readUInt16BE(), 80);
+	socket.write(Buffer.from([5, 0, 0, 1, 127, 0, 0, 1, 0, 80]));
+
+	// Serve a response inside the tunnel without depending on an external server.
+	let request = '';
+	while (!request.endsWith('\r\n\r\n')) {
+		request += (await readBytes(socket, 1)).toString();
+	}
+	assert.ok(request.startsWith('GET /test-path HTTP/1.1\r\n'));
+	socket.end('HTTP/1.1 200 OK\r\nContent-Length: 7\r\nConnection: close\r\n\r\nproxied');
+}
+
+describe('fetch through a SOCKS proxy', function () {
+	for (const addCertificates of [false, true]) {
+		it(`uses a plain SOCKS5 tunnel with additional certificates ${addCertificates ? 'enabled' : 'disabled'}`, async function () {
+			const sockets = new Set<net.Socket>();
+			const errors: unknown[] = [];
+			const proxy = net.createServer(socket => {
+				sockets.add(socket);
+				socket.on('close', () => sockets.delete(socket));
+				void serveSocksRequest(socket).catch(error => {
+					errors.push(error);
+					socket.destroy();
+				});
+			});
+			proxy.listen(0, '127.0.0.1');
+			await once(proxy, 'listening');
+			try {
+				const { port } = proxy.address() as net.AddressInfo;
+				const params = createParams(`socks5://127.0.0.1:${port}`, addCertificates);
+				const { resolveProxyURL } = createProxyResolver(params);
+				const patchedFetch = createFetchPatch(params, globalThis.fetch, resolveProxyURL);
+				const response = await patchedFetch('http://example.test/test-path', { signal: AbortSignal.timeout(1000) });
+				assert.strictEqual(response.status, 200);
+				assert.strictEqual(await response.text(), 'proxied');
+				assert.deepStrictEqual(errors, []);
+			} finally {
+				for (const socket of sockets) {
+					socket.destroy();
+				}
+				await new Promise<void>(resolve => proxy.close(() => resolve()));
+				resetCaches();
+			}
+		});
+	}
+});
diff --git a/tests/test-client/src/proxy.test.ts b/tests/test-client/src/proxy.test.ts
index 370902f..ca44b50 100644
--- a/tests/test-client/src/proxy.test.ts
+++ b/tests/test-client/src/proxy.test.ts
@@ -29,13 +29,19 @@ describe('Proxied client', function () {
 		});
 	});
 
-	it('should use HTTPS proxy for HTTPS connection (fetch)', async function () {
-		const { resolveProxyURL } = vpa.createProxyResolver(tlsProxiedProxyAgentParamsV1);
-		const patchedFetch = vpa.createFetchPatch(tlsProxiedProxyAgentParamsV1, globalThis.fetch, resolveProxyURL);
-		const res = await patchedFetch('https://test-https-server/test-path');
-		assert.strictEqual(res.status, 200);
-		assert.strictEqual((await res.json()).status, 'OK!');
-	});
+	for (const scheme of ['https:', 'HTTPS:']) {
+		it(`should use a ${scheme} proxy for HTTPS connection (fetch)`, async function () {
+			const { resolveProxyURL } = vpa.createProxyResolver(tlsProxiedProxyAgentParamsV1);
+			const patchedFetch = vpa.createFetchPatch(tlsProxiedProxyAgentParamsV1, globalThis.fetch, async url => {
+				const proxyURL = await resolveProxyURL(url);
+				assert.ok(proxyURL?.startsWith('https:'));
+				return proxyURL.replace(/^https:/, scheme);
+			});
+			const res = await patchedFetch('https://test-https-server/test-path');
+			assert.strictEqual(res.status, 200);
+			assert.strictEqual((await res.json()).status, 'OK!');
+		});
+	}
 
 	it('should support basic auth', function () {
 		return testRequest(https, {