diff --git a/.changeset/fluffy-toes-jump.md b/.changeset/fluffy-toes-jump.md
new file mode 100644
index 000000000..daa1737d2
--- /dev/null
+++ b/.changeset/fluffy-toes-jump.md
@@ -0,0 +1,6 @@
+---
+'myst-cli': patch
+'mystmd': patch
+---
+
+Specify port numbers for app and server ports
diff --git a/packages/myst-cli/src/build/site/prepare.ts b/packages/myst-cli/src/build/site/prepare.ts
index 80d1bc9dc..59ab4a508 100644
--- a/packages/myst-cli/src/build/site/prepare.ts
+++ b/packages/myst-cli/src/build/site/prepare.ts
@@ -11,6 +11,8 @@ export type Options = {
   headless?: boolean;
   checkLinks?: boolean;
   yes?: boolean;
+  port?: number;
+  serverPort?: number;
   writeToc?: boolean;
   keepHost?: boolean;
   extraLinkTransformers?: LinkTransformer[];
diff --git a/packages/myst-cli/src/build/site/start.ts b/packages/myst-cli/src/build/site/start.ts
index caee5720e..144b432eb 100644
--- a/packages/myst-cli/src/build/site/start.ts
+++ b/packages/myst-cli/src/build/site/start.ts
@@ -21,8 +21,8 @@ const DEFAULT_START_COMMAND = 'npm run start';
 /**
  * Creates a content server and a websocket that can reload and log messages to the client.
  */
-export async function startContentServer(session: ISession) {
-  const port = await getPort({ port: portNumbers(3100, 3200) });
+export async function startContentServer(session: ISession, opts?: { serverPort?: number }) {
+  const port = opts?.serverPort ?? (await getPort({ port: portNumbers(3100, 3200) }));
   const app = express();
   app.use(cors());
   app.get('/', (req, res) => {
@@ -111,7 +111,7 @@ export async function startServer(
   const mystTemplate = await getMystTemplate(session, opts);
   if (!opts.headless) await installSiteTemplate(session, mystTemplate);
   await buildSite(session, opts);
-  const server = await startContentServer(session);
+  const server = await startContentServer(session, opts);
   const { extraLinkTransformers, extraTransforms, defaultTemplate } = opts;
   if (!opts.buildStatic) {
     watchContent(session, server.reload, {
@@ -130,10 +130,8 @@ export async function startServer(
   session.log.info(
     `\n\n\t✨✨✨  Starting ${mystTemplate.getValidatedTemplateYml().title}  ✨✨✨\n\n`,
   );
-  const port = await getPort({ port: portNumbers(3000, 3100) });
-  const appServer = {
-    port,
-  } as AppServer;
+  const port = opts?.port ?? (await getPort({ port: portNumbers(3000, 3100) }));
+  const appServer = { port } as AppServer;
   await new Promise<void>((resolve) => {
     const start = makeExecutable(
       mystTemplate.getValidatedTemplateYml().build?.start ?? DEFAULT_START_COMMAND,
diff --git a/packages/mystmd/src/options.ts b/packages/mystmd/src/options.ts
index bd3db0379..13ef7d146 100644
--- a/packages/mystmd/src/options.ts
+++ b/packages/mystmd/src/options.ts
@@ -1,4 +1,10 @@
-import { Option } from 'commander';
+import { InvalidArgumentError, Option } from 'commander';
+
+function parseInt(value: any) {
+  const parsedValue = Number.parseInt(value, 10);
+  if (isNaN(parsedValue)) throw new InvalidArgumentError('Not a number.');
+  return parsedValue;
+}
 
 export function makePdfOption(description: string) {
   return new Option('--pdf', description).default(false);
@@ -94,6 +100,23 @@ export function makeHeadlessOption() {
   ).default(false);
 }
 
+export function makePortOption() {
+  return new Option('--port <port>', 'Run the application server from the specified port number')
+    .argParser(parseInt)
+    .env('PORT')
+    .default(undefined);
+}
+
+export function makeServerPortOption() {
+  return new Option(
+    '--server-port <server-port>',
+    'Run the content server from the specified port number',
+  )
+    .argParser(parseInt)
+    .env('SERVER_PORT')
+    .default(undefined);
+}
+
 export function makeYesOption() {
   return new Option('-y, --yes', 'Automatically respond yes to prompts').default(false);
 }
diff --git a/packages/mystmd/src/site.ts b/packages/mystmd/src/site.ts
index ae304b16c..af8746f51 100644
--- a/packages/mystmd/src/site.ts
+++ b/packages/mystmd/src/site.ts
@@ -1,13 +1,20 @@
 import { Command } from 'commander';
 import { Session, startServer } from 'myst-cli';
 import { clirun } from './clirun.js';
-import { makeKeepHostOption, makeHeadlessOption } from './options.js';
+import {
+  makeKeepHostOption,
+  makeHeadlessOption,
+  makePortOption,
+  makeServerPortOption,
+} from './options.js';
 
 export function makeStartCLI(program: Command) {
   const command = new Command('start')
     .description('Start the current project as a website')
     .addOption(makeKeepHostOption())
     .addOption(makeHeadlessOption())
+    .addOption(makePortOption())
+    .addOption(makeServerPortOption())
     .action(clirun(Session, startServer, program));
   return command;
 }