diff --git a/.changeset/many-rivers-hide.md b/.changeset/many-rivers-hide.md
new file mode 100644
index 000000000000..57b131f6203b
--- /dev/null
+++ b/.changeset/many-rivers-hide.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Only apply polyfills where necessary
diff --git a/packages/kit/src/exports/node/polyfills.js b/packages/kit/src/exports/node/polyfills.js
index ba0e49ef8d85..390ab511382a 100644
--- a/packages/kit/src/exports/node/polyfills.js
+++ b/packages/kit/src/exports/node/polyfills.js
@@ -18,6 +18,8 @@ const globals = {
 // exported for dev/preview and node environments
 export function installPolyfills() {
 	for (const name in globals) {
+		if (name in globalThis) continue;
+
 		Object.defineProperty(globalThis, name, {
 			enumerable: true,
 			configurable: true,