diff --git a/packages/directory-plugin/src/index.js b/packages/directory-plugin/src/index.js
index e9ae2eb04..8c1b06fc6 100644
--- a/packages/directory-plugin/src/index.js
+++ b/packages/directory-plugin/src/index.js
@@ -1,8 +1,7 @@
 import { promises as fs } from 'fs';
+import { normalizePath } from 'wmr';
 import path from 'path';
 
-const pathToPosix = p => p.split(path.sep).join(path.posix.sep);
-
 /**
  * @param {object} [options]
  * @param {string} [options.cwd]
@@ -17,7 +16,7 @@ function directoryPlugin(options) {
 			const resolved = await this.resolve(id.slice(4) + '\0', importer, { skipSelf: true });
 
 			if (resolved) {
-				return '\0dir:' + pathToPosix(resolved.id).replace(/\0$/, '');
+				return '\0dir:' + normalizePath(resolved.id).replace(/\0$/, '');
 			}
 		},
 		async load(id) {
diff --git a/packages/sw-plugin/src/index.js b/packages/sw-plugin/src/index.js
index 4e065556b..60acb722a 100644
--- a/packages/sw-plugin/src/index.js
+++ b/packages/sw-plugin/src/index.js
@@ -1,7 +1,6 @@
 import path from 'path';
 import { request } from 'http';
-
-const pathToPosix = p => p.split(path.sep).join(path.posix.sep);
+import { normalizePath } from 'wmr';
 
 /**
  * Service Worker plugin for WMR.
@@ -19,7 +18,7 @@ export default function swPlugin(options) {
 
 	const wmrProxyPlugin = {
 		resolveId(id) {
-			const normalizedId = id[1] + id[2] === ':\\' ? pathToPosix(id.slice(2)) : id;
+			const normalizedId = id[1] + id[2] === ':\\' ? normalizePath(id.slice(2)) : id;
 			if (id.startsWith('/@npm/')) return id;
 			if (!/^\.*\//.test(normalizedId)) return '/@npm/' + id;
 		},
@@ -49,7 +48,7 @@ export default function swPlugin(options) {
 		async resolveId(id, importer) {
 			if (!id.startsWith('sw:')) return;
 			const resolved = await this.resolve(id.slice(3), importer);
-			if (resolved) return `\0sw:${pathToPosix(resolved.id)}`;
+			if (resolved) return `\0sw:${normalizePath(resolved.id)}`;
 		},
 		async load(id) {
 			if (!id.startsWith('\0sw:')) return;
diff --git a/packages/wmr/index.js b/packages/wmr/index.js
index 738d2e0c4..47b0ad92f 100644
--- a/packages/wmr/index.js
+++ b/packages/wmr/index.js
@@ -1,5 +1,14 @@
+import path from 'path';
+
 /**
  * Wrapper for improved intellisense completion
  * @type {typeof import("wmr").defineConfig}
  */
 export const defineConfig = config => config;
+
+/**
+ * Normalize a file path across OSes.
+ * @param {string} file
+ * @returns {string}
+ */
+export const normalizePath = file => file.split(path.win32.sep).join(path.posix.sep);
diff --git a/packages/wmr/src/bundler.js b/packages/wmr/src/bundler.js
index 29657284a..1071253ee 100644
--- a/packages/wmr/src/bundler.js
+++ b/packages/wmr/src/bundler.js
@@ -1,11 +1,9 @@
-import { relative, sep, posix, resolve, dirname } from 'path';
+import { relative, posix, resolve, dirname } from 'path';
 import * as rollup from 'rollup';
 import terser from './plugins/fast-minify.js';
 import totalist from 'totalist';
 import { getPlugins } from './lib/plugins.js';
-
-/** @param {string} p */
-const pathToPosix = p => p.split(sep).join(posix.sep);
+import { normalizePath } from '../index.js';
 
 /** @param {import('wmr').BuildOptions} options */
 export async function bundleProd(options) {
@@ -18,7 +16,7 @@ export async function bundleProd(options) {
 	await totalist(cwd, (rel, abs) => {
 		if (ignore.test(abs)) return;
 		if (!/\.html?/.test(rel)) return;
-		input.push('./' + pathToPosix(relative(root, abs)));
+		input.push('./' + normalizePath(relative(root, abs)));
 	});
 
 	const bundle = await rollup.rollup({
@@ -46,7 +44,7 @@ export async function bundleProd(options) {
 		plugins: [minify && terser({ compress: true, sourcemap })],
 		sourcemap,
 		sourcemapPathTransform(p, mapPath) {
-			let url = pathToPosix(relative(cwd, resolve(dirname(mapPath), p)));
+			let url = normalizePath(relative(cwd, resolve(dirname(mapPath), p)));
 			// strip leading relative path
 			url = url.replace(/^\.\//g, '');
 			// replace internal npm prefix
diff --git a/packages/wmr/src/lib/rollup-plugin-container.js b/packages/wmr/src/lib/rollup-plugin-container.js
index 4d0cc9823..d2203a9f1 100644
--- a/packages/wmr/src/lib/rollup-plugin-container.js
+++ b/packages/wmr/src/lib/rollup-plugin-container.js
@@ -1,18 +1,17 @@
-import { resolve, relative, dirname, sep, posix } from 'path';
+import { resolve, relative, dirname, posix } from 'path';
 import { createHash } from 'crypto';
 import { promises as fs } from 'fs';
 import * as acorn from 'acorn';
 import * as kl from 'kolorist';
 import acornClassFields from 'acorn-class-fields';
 import { debug, formatResolved, formatPath } from './output-utils.js';
+import { normalizePath } from '../../index.js';
 
 // Rollup respects "module", Node 14 doesn't.
 const cjsDefault = m => ('default' in m ? m.default : m);
 /** @type acorn */
 const { Parser } = cjsDefault(acorn);
 
-const toPosixPath = path => path.split(sep).join(posix.sep);
-
 /** Fast splice(x,1) when order doesn't matter (h/t Rich)
  *  @param {Array} array @param {number} index
  */
@@ -54,7 +53,7 @@ export function createPluginContainer(plugins, opts = {}) {
 	const MODULES = opts.modules || new Map();
 
 	function generateFilename({ type, name, fileName, source }) {
-		const posixName = toPosixPath(name);
+		const posixName = normalizePath(name);
 		if (!fileName) {
 			fileName =
 				(type === 'entry' && ctx.outputOptions.file) || ctx.outputOptions[type + 'FileNames'] || '[name][extname]';
@@ -321,7 +320,7 @@ export function createPluginContainer(plugins, opts = {}) {
 					return result;
 				}
 			}
-			return JSON.stringify('/' + fileName.split(sep).join(posix.sep));
+			return JSON.stringify('/' + normalizePath(fileName));
 		}
 	};
 
diff --git a/packages/wmr/src/plugins/url-plugin.js b/packages/wmr/src/plugins/url-plugin.js
index c700fa3e2..ec8cbbaff 100644
--- a/packages/wmr/src/plugins/url-plugin.js
+++ b/packages/wmr/src/plugins/url-plugin.js
@@ -1,5 +1,6 @@
-import { relative, basename, sep, posix } from 'path';
+import { relative, basename } from 'path';
 import { promises as fs } from 'fs';
+import { normalizePath } from '../../index.js';
 
 export const IMPLICIT_URL = /\.(?:png|jpe?g|gif|webp|svg|mp4|webm|ogg|mp3|wav|flac|aac|woff2?|eot|ttf|otf)$/i;
 
@@ -27,7 +28,7 @@ export default function urlPlugin({ inline, cwd } = {}) {
 
 			// In dev mode, we turn the import into an inline module that avoids a network request:
 			if (inline) {
-				const url = '/' + relative(cwd, resolved.id).replace(/^\./, '').split(sep).join(posix.sep) + '?asset';
+				const url = '/' + normalizePath(relative(cwd, resolved.id).replace(/^\./, '')) + '?asset';
 				return {
 					id: escapeUrl(`data:text/javascript,export default${JSON.stringify(url)}`),
 					external: true
diff --git a/packages/wmr/src/plugins/wmr/styles-plugin.js b/packages/wmr/src/plugins/wmr/styles-plugin.js
index cac5fa403..d3a0b8adf 100644
--- a/packages/wmr/src/plugins/wmr/styles-plugin.js
+++ b/packages/wmr/src/plugins/wmr/styles-plugin.js
@@ -1,5 +1,6 @@
 import { promises as fs } from 'fs';
 import { basename, dirname, relative, resolve, sep, posix } from 'path';
+import { normalizePath } from '../../../index.js';
 import { transformCssImports } from '../../lib/transform-css-imports.js';
 import { transformCss } from '../../lib/transform-css.js';
 
@@ -20,7 +21,7 @@ export function processSass(sass) {
  */
 export async function modularizeCss(css, id, mappings = [], idAbsolute) {
 	// normalize to posix id for consistent hashing
-	if (id.match(/^[^/]*\\/)) id = id.split(sep).join(posix.sep);
+	if (id.match(/^[^/]*\\/)) id = normalizePath(id);
 
 	const suffix = '_' + hash(id);
 
@@ -123,7 +124,7 @@ export default function wmrStylesPlugin({ cwd, hot, fullPath, production } = {})
 			const isModular = /\.module\.(css|s[ac]ss)$/.test(id);
 
 			let idRelative = cwd ? relative(cwd || '', resolve(cwd, id)) : multiRelative(cwds, id);
-			if (idRelative.match(/^[^/]*\\/)) idRelative = idRelative.split(sep).join(posix.sep);
+			if (idRelative.match(/^[^/]*\\/)) idRelative = normalizePath(idRelative);
 
 			const mappings = [];
 			if (isModular) {
diff --git a/packages/wmr/src/wmr-middleware.js b/packages/wmr/src/wmr-middleware.js
index 5664908f0..94eb3659e 100644
--- a/packages/wmr/src/wmr-middleware.js
+++ b/packages/wmr/src/wmr-middleware.js
@@ -11,6 +11,7 @@ import { getMimeType } from './lib/mimetypes.js';
 import { debug, formatPath } from './lib/output-utils.js';
 import { getPlugins } from './lib/plugins.js';
 import { watch } from './lib/fs-watcher.js';
+import { normalizePath } from '../index.js';
 
 const NOOP = () => {};
 
@@ -96,7 +97,7 @@ export default function wmrMiddleware(options) {
 	watcher.on('change', filename => {
 		NonRollup.watchChange(resolve(cwd, filename));
 		// normalize paths to 'nix:
-		filename = filename.split(sep).join(posix.sep);
+		filename = normalizePath(filename);
 
 		// Delete any generated CSS Modules mapping modules:
 		const suffix = /\.module\.(css|s[ac]ss)$/.test(filename) ? '.js' : '';
@@ -158,7 +159,7 @@ export default function wmrMiddleware(options) {
 		let file = resolve(cwd, osPath);
 
 		// Rollup-style CWD-relative Unix-normalized path "id":
-		let id = relative(cwd, file).replace(/^\.\//, '').replace(/^[\0]/, '').split(sep).join(posix.sep);
+		let id = normalizePath(relative(cwd, file).replace(/^\.\//, '').replace(/^[\0]/, ''));
 
 		// add back any prefix if there was one:
 		file = prefix + file;
@@ -319,7 +320,7 @@ export const TRANSFORMS = {
 					if (resolved) {
 						spec = typeof resolved == 'object' ? resolved.id : resolved;
 						if (/^(\/|\\|[a-z]:\\)/i.test(spec)) {
-							spec = relative(dirname(file), spec).split(sep).join(posix.sep);
+							spec = normalizePath(relative(dirname(file), spec));
 							if (!/^\.?\.?\//.test(spec)) {
 								spec = './' + spec;
 							}
@@ -330,7 +331,7 @@ export const TRANSFORMS = {
 								return spec;
 							}
 
-							spec = relative(cwd, spec).split(sep).join(posix.sep);
+							spec = normalizePath(relative(cwd, spec));
 							if (!/^(\/|[\w-]+:)/.test(spec)) spec = `/${spec}`;
 							return spec;
 						}
@@ -340,7 +341,7 @@ export const TRANSFORMS = {
 					spec = spec.replace(/^\0?([a-z-]+):(.+)$/, (s, prefix, spec) => {
 						// \0abc:/abs/disk/path --> /@abc/cwd-relative-path
 						if (spec[0] === '/' || spec[0] === sep) {
-							spec = relative(cwd, spec).split(sep).join(posix.sep);
+							spec = normalizePath(relative(cwd, spec));
 						}
 						return '/@' + prefix + '/' + spec;
 					});
diff --git a/packages/wmr/types.d.ts b/packages/wmr/types.d.ts
index 2ec6d1309..3abffb62f 100644
--- a/packages/wmr/types.d.ts
+++ b/packages/wmr/types.d.ts
@@ -68,6 +68,8 @@ declare module 'wmr' {
 	export function defineConfig<
 		T extends Partial<Options> | ((options: Options) => void | Partial<Options> | Promise<void | Partial<Options>>)
 	>(options: T): T;
+
+	export function normalizePath(path: string): string;
 }
 
 // Declarations used by WMR-based applications