diff --git a/.changeset/six-eels-crash.md b/.changeset/six-eels-crash.md
new file mode 100644
index 000000000000..15a2a0ae72c3
--- /dev/null
+++ b/.changeset/six-eels-crash.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+chore: update Undici to 5.21.0
diff --git a/package.json b/package.json
index 05b20f6a8b36..530a02c2375a 100644
--- a/package.json
+++ b/package.json
@@ -41,7 +41,7 @@
 		"tiny-glob": "^0.2.9",
 		"typescript": "^4.9.4"
 	},
-	"packageManager": "pnpm@7.29.1",
+	"packageManager": "pnpm@7.29.3",
 	"engines": {
 		"pnpm": "^7.0.0"
 	},
diff --git a/packages/kit/package.json b/packages/kit/package.json
index 88bacabb6f5d..7bd120e623fd 100644
--- a/packages/kit/package.json
+++ b/packages/kit/package.json
@@ -22,7 +22,7 @@
 		"set-cookie-parser": "^2.5.1",
 		"sirv": "^2.0.2",
 		"tiny-glob": "^0.2.9",
-		"undici": "5.20.0"
+		"undici": "5.21.0"
 	},
 	"devDependencies": {
 		"@playwright/test": "^1.29.2",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 3482d10ada4f..44ffd2bfdd47 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -287,7 +287,7 @@ importers:
       svelte-preprocess: ^5.0.0
       tiny-glob: ^0.2.9
       typescript: ^4.9.4
-      undici: 5.20.0
+      undici: 5.21.0
       uvu: ^0.5.6
       vite: ^4.1.1
     dependencies:
@@ -303,7 +303,7 @@ importers:
       set-cookie-parser: 2.5.1
       sirv: 2.0.2
       tiny-glob: 0.2.9
-      undici: 5.20.0
+      undici: 5.21.0
     devDependencies:
       '@playwright/test': 1.29.2
       '@types/connect': 3.4.35
@@ -5235,8 +5235,8 @@ packages:
       which-boxed-primitive: 1.0.2
     dev: true
 
-  /undici/5.20.0:
-    resolution: {integrity: sha512-J3j60dYzuo6Eevbawwp1sdg16k5Tf768bxYK4TUJRH7cBM4kFCbf3mOnM/0E3vQYXvpxITbbWmBafaDbxLDz3g==}
+  /undici/5.21.0:
+    resolution: {integrity: sha512-HOjK8l6a57b2ZGXOcUsI5NLfoTrfmbOl90ixJDl0AEFG4wgHNDQxtZy15/ZQp7HhjkpaGlp/eneMgtsu1dIlUA==}
     engines: {node: '>=12.18'}
     dependencies:
       busboy: 1.6.0