diff --git a/.changeset/clear-deer-doubt.md b/.changeset/clear-deer-doubt.md
new file mode 100644
index 000000000000..572d9fd9b265
--- /dev/null
+++ b/.changeset/clear-deer-doubt.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: ensure CSS URL references are absolute when `paths.relative` is `false`
diff --git a/packages/kit/src/exports/vite/index.js b/packages/kit/src/exports/vite/index.js
index 5f2c1e629371..bd3af895b4e0 100644
--- a/packages/kit/src/exports/vite/index.js
+++ b/packages/kit/src/exports/vite/index.js
@@ -981,7 +981,7 @@ async function kit({ svelte_config }) {
 
 				if (is_build) {
 					const ssr = /** @type {boolean} */ (config.build?.ssr);
-					const prefix = `${kit.appDir}/immutable`;
+					const app_immutable = `${kit.appDir}/immutable`;
 
 					/** @type {Record<string, string>} */
 					const input = {};
@@ -1071,9 +1071,9 @@ async function kit({ svelte_config }) {
 
 					/** @type {string} */
 					const base = (kit.paths.assets || kit.paths.base) + '/';
-					const root_to_assets = prefix + '/assets/';
+					const root_to_assets = app_immutable + '/assets/';
 					const assets_to_root =
-						prefix
+						app_immutable
 							.split('/')
 							.map(() => '..')
 							.join('/') + '/../';
@@ -1099,9 +1099,11 @@ async function kit({ svelte_config }) {
 								output: {
 									format: inline ? 'iife' : 'esm',
 									name: `__sveltekit_${version_hash}.app`,
-									entryFileNames: ssr ? '[name].js' : `${prefix}/[name].[hash].${ext}`,
-									chunkFileNames: ssr ? 'chunks/[name].js' : `${prefix}/chunks/[hash].${ext}`,
-									assetFileNames: `${prefix}/assets/[name].[hash][extname]`,
+									entryFileNames: ssr ? '[name].js' : `${app_immutable}/[name].[hash].${ext}`,
+									chunkFileNames: ssr
+										? 'chunks/[name].js'
+										: `${app_immutable}/chunks/[hash].${ext}`,
+									assetFileNames: `${app_immutable}/assets/[name].[hash][extname]`,
 									hoistTransitiveImports: false,
 									sourcemapIgnoreList,
 									inlineDynamicImports: is_rolldown ? undefined : !split
@@ -1129,9 +1131,9 @@ async function kit({ svelte_config }) {
 						worker: {
 							rollupOptions: {
 								output: {
-									entryFileNames: `${prefix}/workers/[name]-[hash].js`,
-									chunkFileNames: `${prefix}/workers/chunks/[hash].js`,
-									assetFileNames: `${prefix}/workers/assets/[name]-[hash][extname]`,
+									entryFileNames: `${app_immutable}/workers/[name]-[hash].js`,
+									chunkFileNames: `${app_immutable}/workers/chunks/[hash].js`,
+									assetFileNames: `${app_immutable}/workers/assets/[name]-[hash][extname]`,
 									hoistTransitiveImports: false
 								}
 							}
@@ -1151,6 +1153,11 @@ async function kit({ svelte_config }) {
 									return { relative };
 								}
 
+								if (!relative) return;
+
+								// ensure assets loaded by CSS files are loaded relative to the
+								// CSS file rather than the default of relative to the root
+
 								// _app/immutable/assets files
 								if (filename.startsWith(root_to_assets)) {
 									return `./${filename.slice(root_to_assets.length)}`;
diff --git a/packages/kit/test/apps/options/package.json b/packages/kit/test/apps/options/package.json
index 3e7c004eea33..85c9ed6c8d31 100644
--- a/packages/kit/test/apps/options/package.json
+++ b/packages/kit/test/apps/options/package.json
@@ -10,7 +10,10 @@
 		"check": "svelte-kit sync && tsc && svelte-check",
 		"test": "pnpm test:dev && pnpm test:build",
 		"test:dev": "DEV=true playwright test && DEV=true PATHS_ASSETS=https://cdn.example.com/stuff playwright test",
-		"test:build": "playwright test && PATHS_ASSETS=https://cdn.example.com/stuff playwright test",
+		"test:paths-relative:build": "playwright test",
+		"test:paths-absolute:build": "PATHS_RELATIVE=false playwright test",
+		"test:paths-assets:build": "PATHS_ASSETS=https://cdn.example.com/stuff playwright test",
+		"test:build": "pnpm test:paths-relative:build && pnpm test:paths-absolute:build && pnpm test:paths-assets:build",
 		"test:unit": "vitest run --config vite.custom.config.js",
 		"test:server-side-route-resolution:dev": "rm -rf test/errors.json && DEV=true ROUTER_RESOLUTION=server playwright test",
 		"test:server-side-route-resolution:build": "rm -rf test/errors.json && ROUTER_RESOLUTION=server playwright test"
diff --git a/packages/kit/test/apps/options/playwright.config.js b/packages/kit/test/apps/options/playwright.config.js
index cdb691f211a8..975bba3b3305 100644
--- a/packages/kit/test/apps/options/playwright.config.js
+++ b/packages/kit/test/apps/options/playwright.config.js
@@ -9,7 +9,8 @@ export default defineConfig({
 		command: process.env.DEV ? `pnpm dev` : `pnpm build && pnpm preview`,
 		env: {
 			ROUTER_RESOLUTION: process.env.ROUTER_RESOLUTION ?? 'client',
-			PATHS_ASSETS: process.env.PATHS_ASSETS ?? ''
+			PATHS_ASSETS: process.env.PATHS_ASSETS ?? '',
+			PATHS_RELATIVE: process.env.PATHS_RELATIVE ?? 'true'
 		}
 	}
 });
diff --git a/packages/kit/test/apps/options/svelte.config.js b/packages/kit/test/apps/options/svelte.config.js
index da8cebe35818..271373259438 100644
--- a/packages/kit/test/apps/options/svelte.config.js
+++ b/packages/kit/test/apps/options/svelte.config.js
@@ -33,7 +33,8 @@ const config = {
 		paths: {
 			base: '/path-base',
 			// @ts-expect-error our env var string can't match the https template literal
-			assets: process.env.PATHS_ASSETS || ''
+			assets: process.env.PATHS_ASSETS || '',
+			relative: process.env.PATHS_RELATIVE !== 'false'
 		},
 		env: {
 			dir: './env-dir',
diff --git a/packages/kit/test/apps/options/test/paths-assets.test.js b/packages/kit/test/apps/options/test/paths.test.js
similarity index 100%
rename from packages/kit/test/apps/options/test/paths-assets.test.js
rename to packages/kit/test/apps/options/test/paths.test.js
diff --git a/packages/kit/test/apps/options/test/test.js b/packages/kit/test/apps/options/test/test.js
index ceefb0383b18..a7356b5fec04 100644
--- a/packages/kit/test/apps/options/test/test.js
+++ b/packages/kit/test/apps/options/test/test.js
@@ -5,7 +5,7 @@ import { test } from '../../../utils.js';
 
 test.describe.configure({ mode: 'parallel' });
 
-test.skip(!!process.env.PATHS_ASSETS);
+test.skip(!!process.env.PATHS_ASSETS || process.env.PATHS_RELATIVE === 'false');
 
 test.describe('CSP', () => {
 	test('blocks script from external site', async ({ page, start_server }) => {