diff --git a/.changeset/wise-adults-pull.md b/.changeset/wise-adults-pull.md
new file mode 100644
index 000000000000..5bde68e010bb
--- /dev/null
+++ b/.changeset/wise-adults-pull.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[fix] don't polyfill undici if using Deno or Bun
diff --git a/packages/kit/src/core/prerender/prerender.js b/packages/kit/src/core/prerender/prerender.js
index 65ece5d73de9..aa3e732c6835 100644
--- a/packages/kit/src/core/prerender/prerender.js
+++ b/packages/kit/src/core/prerender/prerender.js
@@ -1,8 +1,9 @@
 import { readFileSync, writeFileSync } from 'fs';
 import { dirname, join } from 'path';
 import { pathToFileURL, URL } from 'url';
-import { mkdirp, posixify, walk } from '../../utils/filesystem.js';
 import { installPolyfills } from '../../exports/node/polyfills.js';
+import { mkdirp, posixify, walk } from '../../utils/filesystem.js';
+import { should_polyfill } from '../../utils/platform.js';
 import { is_root_relative, resolve } from '../../utils/url.js';
 import { queue } from './queue.js';
 import { crawl } from './crawl.js';
@@ -89,7 +90,9 @@ export async function prerender() {
 		verbose: verbose === 'true'
 	});
 
-	installPolyfills();
+	if (should_polyfill) {
+		installPolyfills();
+	}
 
 	const server_root = join(config.outDir, 'output');
 
diff --git a/packages/kit/src/exports/node/polyfills.js b/packages/kit/src/exports/node/polyfills.js
index ba0e49ef8d85..88805160b547 100644
--- a/packages/kit/src/exports/node/polyfills.js
+++ b/packages/kit/src/exports/node/polyfills.js
@@ -16,6 +16,7 @@ const globals = {
 };
 
 // exported for dev/preview and node environments
+// TODO: remove this once we only support Node 18.11+ (the version multipart/form-data was added)
 export function installPolyfills() {
 	for (const name in globals) {
 		Object.defineProperty(globalThis, name, {
diff --git a/packages/kit/src/exports/vite/dev/index.js b/packages/kit/src/exports/vite/dev/index.js
index 545af8d1f464..571db3d223b9 100644
--- a/packages/kit/src/exports/vite/dev/index.js
+++ b/packages/kit/src/exports/vite/dev/index.js
@@ -8,6 +8,7 @@ import { getRequest, setResponse } from '../../../exports/node/index.js';
 import { installPolyfills } from '../../../exports/node/polyfills.js';
 import { coalesce_to_error } from '../../../utils/error.js';
 import { posixify, resolve_entry, to_fs } from '../../../utils/filesystem.js';
+import { should_polyfill } from '../../../utils/platform.js';
 import { load_error_page, load_template } from '../../../core/config/index.js';
 import { SVELTE_KIT_ASSETS } from '../../../constants.js';
 import * as sync from '../../../core/sync/sync.js';
@@ -24,7 +25,9 @@ const cwd = process.cwd();
  * @return {Promise<Promise<() => void>>}
  */
 export async function dev(vite, vite_config, svelte_config) {
-	installPolyfills();
+	if (should_polyfill) {
+		installPolyfills();
+	}
 
 	sync.init(svelte_config, vite_config.mode);
 
diff --git a/packages/kit/src/exports/vite/preview/index.js b/packages/kit/src/exports/vite/preview/index.js
index 45ac7344b018..b62db099cc6c 100644
--- a/packages/kit/src/exports/vite/preview/index.js
+++ b/packages/kit/src/exports/vite/preview/index.js
@@ -2,10 +2,11 @@ import fs from 'fs';
 import { join } from 'path';
 import sirv from 'sirv';
 import { pathToFileURL } from 'url';
+import { loadEnv, normalizePath } from 'vite';
 import { getRequest, setResponse } from '../../../exports/node/index.js';
 import { installPolyfills } from '../../../exports/node/polyfills.js';
 import { SVELTE_KIT_ASSETS } from '../../../constants.js';
-import { loadEnv, normalizePath } from 'vite';
+import { should_polyfill } from '../../../utils/platform.js';
 import { not_found } from '../utils.js';
 
 /** @typedef {import('http').IncomingMessage} Req */
@@ -21,7 +22,9 @@ import { not_found } from '../utils.js';
  * @param {import('types').ValidatedConfig} svelte_config
  */
 export async function preview(vite, vite_config, svelte_config) {
-	installPolyfills();
+	if (should_polyfill) {
+		installPolyfills();
+	}
 
 	const { paths } = svelte_config.kit;
 	const base = paths.base;
diff --git a/packages/kit/src/utils/platform.js b/packages/kit/src/utils/platform.js
new file mode 100644
index 000000000000..e32dd5f5abce
--- /dev/null
+++ b/packages/kit/src/utils/platform.js
@@ -0,0 +1 @@
+export const should_polyfill = typeof Deno === 'undefined' && typeof Bun === 'undefined';
diff --git a/packages/kit/types/internal.d.ts b/packages/kit/types/internal.d.ts
index f45b76390da2..4d558c88e58a 100644
--- a/packages/kit/types/internal.d.ts
+++ b/packages/kit/types/internal.d.ts
@@ -389,4 +389,6 @@ declare global {
 	const __SVELTEKIT_BROWSER__: boolean;
 	const __SVELTEKIT_DEV__: boolean;
 	const __SVELTEKIT_EMBEDDED__: boolean;
+	var Bun: object;
+	var Deno: object;
 }