diff --git a/core/packages/gax/test/showcase-server/src/index.ts b/core/packages/gax/test/showcase-server/src/index.ts
index a7f904cc6bff..8f3a5110fff3 100644
--- a/core/packages/gax/test/showcase-server/src/index.ts
+++ b/core/packages/gax/test/showcase-server/src/index.ts
@@ -32,8 +32,16 @@ function sleep(timeoutMs: number) {
 export class ShowcaseServer {
   server: execa.ExecaChildProcess | undefined;
 
-  async start() {
-    const testDir = path.join(process.cwd(), '.showcase-server-dir');
+  /**
+   * Starts the gapic-showcase server.
+   * @param opts Optional configuration for the server:
+   *  - tls: If true, starts the server with TLS enabled.
+   *  - port: The port to bind the server to (e.g. ":7443").
+   *  - caCertOutputFile: Path where the server should write its CA cert (when TLS is enabled).
+   */
+  async start(opts?: { tls?: boolean; port?: string; caCertOutputFile?: string }) {
+    const cwd = process.cwd();
+    const testDir = path.join(cwd, '.showcase-server-dir');
     const platform = process.platform;
     const arch = process.arch === 'x64' ? 'amd64' : process.arch;
     const showcaseVersion = process.env['SHOWCASE_VERSION'] || '0.36.2';
@@ -41,14 +49,31 @@ export class ShowcaseServer {
     const fallbackServerUrl = `https://github.com/googleapis/gapic-showcase/releases/download/v${showcaseVersion}/${tarballFilename}`;
     const binaryName = './gapic-showcase';
 
+    let resolvedCaCertPath = '';
+    if (opts?.caCertOutputFile) {
+      resolvedCaCertPath = path.resolve(cwd, opts.caCertOutputFile);
+    }
+
     await fsp.rm(testDir, {recursive: true, force: true});
     await mkdir(testDir);
-    process.chdir(testDir);
     console.log(`Server will be run from ${testDir}.`);
 
     await download(fallbackServerUrl, testDir);
-    await execa('tar', ['xzf', tarballFilename]);
-    const childProcess = execa(binaryName, ['run'], {
+    await execa('tar', ['xzf', tarballFilename], {cwd: testDir});
+
+    const args = ['run'];
+    // Pass additional options to gapic-showcase based on opts
+    if (opts?.tls) {
+      args.push('--tls');
+    }
+    if (opts?.port) {
+      args.push('--port', opts.port);
+    }
+    if (resolvedCaCertPath) {
+      args.push('--ca-cert-output-file', resolvedCaCertPath);
+    }
+
+    const childProcess = execa(binaryName, args, {
       cwd: testDir,
       stdio: 'inherit',
     });
@@ -68,6 +93,40 @@ export class ShowcaseServer {
     );
 
     this.server = childProcess;
+
+    if (resolvedCaCertPath) {
+      const dir = path.dirname(resolvedCaCertPath);
+      const filename = path.basename(resolvedCaCertPath);
+
+      const pemExists = await new Promise<boolean>((resolve) => {
+        if (fs.existsSync(resolvedCaCertPath)) {
+          return resolve(true);
+        }
+
+        const timeoutId = setTimeout(() => {
+          watcher.close();
+          resolve(false);
+        }, 15000);
+
+        const watcher = fs.watch(dir, (eventType, triggeredFilename) => {
+          if (triggeredFilename === filename || fs.existsSync(resolvedCaCertPath)) {
+            clearTimeout(timeoutId);
+            watcher.close();
+            // Wait a few milliseconds to ensure showcase has finished writing the file.
+            setTimeout(() => {
+              resolve(true);
+            }, 500);
+          }
+        });
+      });
+
+      if (!pemExists) {
+        throw new Error(`CA Certificate file not found at ${resolvedCaCertPath}`);
+      }
+
+      return fs.readFileSync(resolvedCaCertPath);
+    }
+    return undefined;
   }
 
   stop() {
diff --git a/core/packages/gax/test/test-application/src/index.ts b/core/packages/gax/test/test-application/src/index.ts
index d6ab4154e17b..c0035e1de447 100644
--- a/core/packages/gax/test/test-application/src/index.ts
+++ b/core/packages/gax/test/test-application/src/index.ts
@@ -19,6 +19,7 @@ import {EchoClient, SequenceServiceClient, protos} from 'showcase-echo-client';
 import {ShowcaseServer} from 'showcase-server';
 import * as assert from 'assert';
 import {promises as fsp} from 'fs';
+import {runPqcComplianceTests} from './pqc-test';
 import * as path from 'path';
 import {
   protobuf,
@@ -2910,6 +2911,8 @@ async function main() {
   } finally {
     showcaseServer.stop();
   }
+  // Run PQC tests with a different showcase server because setup with a certificate is required.
+  await runPqcComplianceTests()
 }
 
 main();
diff --git a/core/packages/gax/test/test-application/src/pqc-test.ts b/core/packages/gax/test/test-application/src/pqc-test.ts
new file mode 100644
index 000000000000..e506b0b8767c
--- /dev/null
+++ b/core/packages/gax/test/test-application/src/pqc-test.ts
@@ -0,0 +1,211 @@
+/**
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as fs from 'fs';
+import * as https from 'https';
+import * as path from 'path';
+import * as assert from 'assert';
+import {grpc, GoogleAuth, googleAuthLibrary} from 'google-gax';
+import {EchoClient} from 'showcase-echo-client';
+import {ShowcaseServer} from 'showcase-server';
+
+import * as os from 'os';
+import * as tls from 'tls';
+
+async function grpcPqcTest(pemBuffer: Buffer, port: number) {
+  let negotiatedGroupGrpc: string | undefined;
+  let clientSupportedGroupsGrpc: string | undefined;
+
+  const interceptor = (options: any, nextCall: any) => {
+    return new grpc.InterceptingCall(nextCall(options), {
+      start: (metadata: any, listener: any, next: any) => {
+        next(metadata, {
+          onReceiveMetadata: (receivedMetadata: any, nextListener: any) => {
+            const group = receivedMetadata.get('x-showcase-tls-group');
+            if (group && group.length > 0) {
+              negotiatedGroupGrpc = group[0].toString();
+            }
+            const supportedGroups = receivedMetadata.get(
+              'x-showcase-tls-client-supported-groups'
+            );
+            if (supportedGroups && supportedGroups.length > 0) {
+              clientSupportedGroupsGrpc = supportedGroups[0].toString();
+            }
+            nextListener(receivedMetadata);
+          },
+        });
+      },
+    });
+  };
+
+  const grpcClientOpts = {
+    grpc,
+    sslCreds: grpc.credentials.createSsl(pemBuffer),
+    servicePath: 'localhost',
+    port: port,
+  };
+
+  const grpcClient = new EchoClient(grpcClientOpts);
+
+  const [responseGrpc] = await grpcClient.echo(
+    { content: 'grpc-pqc-test' },
+    {
+      otherArgs: {
+        options: {
+          interceptors: [interceptor],
+        },
+      },
+    }
+  );
+
+  assert.strictEqual(responseGrpc.content, 'grpc-pqc-test');
+  assert.ok(
+    negotiatedGroupGrpc,
+    'Expected negotiated TLS group in gRPC response metadata'
+  );
+  assert.strictEqual(negotiatedGroupGrpc, 'X25519MLKEM768');
+  assert.ok(
+    clientSupportedGroupsGrpc,
+    'Expected client supported groups in gRPC response metadata'
+  );
+  assert.ok(
+    clientSupportedGroupsGrpc.includes('X25519MLKEM768'),
+    'Expected client to include X25519MLKEM768 in supported groups'
+  );
+}
+
+async function httpRestFallbackPqcTest(pemBuffer: Buffer, port: number) {
+  let negotiatedGroupRest: string | undefined;
+  let clientSupportedGroupsRest: string | undefined;
+
+  const auth = new GoogleAuth({
+    authClient: new googleAuthLibrary.PassThroughClient(),
+  });
+
+  const originalFetch = auth.fetch.bind(auth);
+  (auth as any).fetch = async (url: string, opts: any) => {
+    if (url.startsWith('https:')) {
+      opts.agent = new https.Agent({
+        ca: pemBuffer,
+        keepAlive: true,
+      });
+    }
+    const res = await originalFetch(url, opts);
+    // res.headers is a Headers instance (with .get()) in standard Fetch API and a plain object in other HTTP clients.
+    const group =
+      typeof res.headers.get === 'function'
+        ? res.headers.get('x-showcase-tls-group')
+        : (res.headers as any)['x-showcase-tls-group'];
+    if (group) {
+      negotiatedGroupRest = group;
+    }
+    const supportedGroups =
+      typeof res.headers.get === 'function'
+        ? res.headers.get('x-showcase-tls-client-supported-groups')
+        : (res.headers as any)['x-showcase-tls-client-supported-groups'];
+    if (supportedGroups) {
+      clientSupportedGroupsRest = supportedGroups;
+    }
+    return res;
+  };
+
+  const restClientOpts = {
+    fallback: true,
+    protocol: 'https',
+    servicePath: 'localhost',
+    port: port,
+    auth: auth,
+  };
+
+  const restClient = new EchoClient(restClientOpts);
+  const [responseRest] = await restClient.echo({ content: 'rest-pqc-test' });
+
+  assert.strictEqual(responseRest.content, 'rest-pqc-test');
+  assert.ok(
+    negotiatedGroupRest,
+    'Expected negotiated TLS group in REST response headers'
+  );
+  assert.strictEqual(negotiatedGroupRest, 'X25519MLKEM768');
+  assert.ok(
+    clientSupportedGroupsRest,
+    'Expected client supported groups in REST response headers'
+  );
+  assert.ok(
+    clientSupportedGroupsRest.includes('X25519MLKEM768'),
+    'Expected client to include X25519MLKEM768 in supported groups'
+  );
+}
+
+/**
+ * Tests Post Quantum Cryptography (PQC) using the specified CA cert and port.
+ * It verifies both gRPC and HTTP/REST clients by inspecting the negotiated TLS group.
+ * @param pemBuffer The CA certificate buffer.
+ * @param port The port the TLS showcase server is listening on.
+ */
+async function testPqc(pemBuffer: Buffer, port: number) {
+  await grpcPqcTest(pemBuffer, port);
+  await httpRestFallbackPqcTest(pemBuffer, port);
+}
+
+/**
+ * Spins up a TLS-enabled showcase server and runs the PQC compliance tests.
+ * Cleans up the generated certificate file after the tests complete.
+ */
+export async function runPqcComplianceTests() {
+  const [major, minor] = process.version.replace('v', '').split('.').map(Number);
+  if (major < 22 || (major === 22 && minor < 20)) {
+    console.log(
+      `skipping PQC compliance tests because node version is ${process.version}`
+    );
+    return;
+  }
+
+  const originalShowcaseVersion = process.env['SHOWCASE_VERSION'];
+  process.env['SHOWCASE_VERSION'] = '0.41.1';
+  const showcaseServerTls = new ShowcaseServer();
+  const tlsPort = 7443;
+  const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'pqc-test-'));
+  const caCertOutputFile = path.join(tempDir, 'showcase.pem');
+  const pemPath = caCertOutputFile;
+
+  try {
+    if (fs.existsSync(pemPath)) {
+      fs.unlinkSync(pemPath);
+    }
+
+    const pemBuffer = await showcaseServerTls.start({
+      tls: true,
+      port: `:${tlsPort}`,
+      caCertOutputFile: caCertOutputFile,
+    });
+
+    if (!pemBuffer) {
+      throw new Error('Expected showcase server to return CA certificate buffer');
+    }
+
+    await testPqc(pemBuffer, tlsPort);
+  } finally {
+    showcaseServerTls.stop();
+    if (originalShowcaseVersion === undefined) {
+      delete process.env['SHOWCASE_VERSION'];
+    } else {
+      process.env['SHOWCASE_VERSION'] = originalShowcaseVersion;
+    }
+    if (fs.existsSync(tempDir)) {
+      fs.rmSync(tempDir, {recursive: true, force: true});
+    }
+  }
+}