diff --git a/packages/pluginutils/README.md b/packages/pluginutils/README.md
index 7475be7af..f02dda33a 100755
--- a/packages/pluginutils/README.md
+++ b/packages/pluginutils/README.md
@@ -50,9 +50,9 @@ export default function myPlugin(options = {}) {
   return {
     resolveId(code, id) {
       // only adds an extension if there isn't one already
-      id = addExtension(id); // `foo` -> `foo.js`, `foo.js -> foo.js`
-      id = addExtension(id, '.myext'); // `foo` -> `foo.myext`, `foo.js -> `foo.js`
-    }
+      id = addExtension(id); // `foo` -> `foo.js`, `foo.js` -> `foo.js`
+      id = addExtension(id, '.myext'); // `foo` -> `foo.myext`, `foo.js` -> `foo.js`
+    },
   };
 }
 ```
@@ -88,9 +88,9 @@ export default function myPlugin(options = {}) {
         },
         leave(node) {
           if (node.scope) scope = scope.parent;
-        }
+        },
       });
-    }
+    },
   };
 }
 ```
@@ -126,7 +126,7 @@ import { createFilter } from '@rollup/pluginutils';
 export default function myPlugin(options = {}) {
   // assume that the myPlugin accepts options of `options.include` and `options.exclude`
   var filter = createFilter(options.include, options.exclude, {
-    resolve: '/my/base/dir'
+    resolve: '/my/base/dir',
   });
 
   return {
@@ -134,7 +134,7 @@ export default function myPlugin(options = {}) {
       if (!filter(id)) return;
 
       // proceed with the transformation...
-    }
+    },
   };
 }
 ```
@@ -160,14 +160,14 @@ import { dataToEsm } from '@rollup/pluginutils';
 const esModuleSource = dataToEsm(
   {
     custom: 'data',
-    to: ['treeshake']
+    to: ['treeshake'],
   },
   {
     compact: false,
     indent: '\t',
     preferConst: false,
     objectShorthand: false,
-    namedExports: true
+    namedExports: true,
   }
 );
 /*
@@ -207,11 +207,11 @@ export default function myPlugin(options = {}) {
           if (node.type === 'VariableDeclarator') {
             const declaredNames = extractAssignedNames(node.id);
             // do something with the declared names
-            // e.g. for `const {x, y: z} = ... => declaredNames = ['x', 'z']
+            // e.g. for `const {x, y: z} = ...` => declaredNames = ['x', 'z']
           }
-        }
+        },
       });
-    }
+    },
   };
 }
 ```
@@ -232,6 +232,22 @@ makeLegalIdentifier('foo-bar'); // 'foo_bar'
 makeLegalIdentifier('typeof'); // '_typeof'
 ```
 
+### normalizePath
+
+Converts path separators to forward slash.
+
+Parameters: `(filename: String)`<br>
+Returns: `String`
+
+#### Usage
+
+```js
+import { normalizePath } from '@rollup/pluginutils';
+
+normalizePath('foo\\bar'); // 'foo/bar'
+normalizePath('foo/bar'); // 'foo/bar'
+```
+
 ## Meta
 
 [CONTRIBUTING](/.github/CONTRIBUTING.md)
diff --git a/packages/pluginutils/src/createFilter.ts b/packages/pluginutils/src/createFilter.ts
index 09ca831d2..67d65fce6 100755
--- a/packages/pluginutils/src/createFilter.ts
+++ b/packages/pluginutils/src/createFilter.ts
@@ -1,10 +1,11 @@
-import { resolve, sep, posix, isAbsolute } from 'path';
+import { resolve, posix, isAbsolute } from 'path';
 
 import pm from 'picomatch';
 
 import { CreateFilter } from '../types';
 
 import ensureArray from './utils/ensureArray';
+import normalizePath from './normalizePath';
 
 function getMatcherString(id: string, resolutionBase: string | false | null | undefined) {
   if (resolutionBase === false || isAbsolute(id) || id.startsWith('*')) {
@@ -12,9 +13,7 @@ function getMatcherString(id: string, resolutionBase: string | false | null | un
   }
 
   // resolve('') is valid and will default to process.cwd()
-  const basePath = resolve(resolutionBase || '')
-    .split(sep)
-    .join('/')
+  const basePath = normalizePath(resolve(resolutionBase || ''))
     // escape all possible (posix + win) path characters that might interfere with regex
     .replace(/[-^$*+?.()|[\]{}]/g, '\\$&');
   // Note that we use posix.join because:
@@ -48,7 +47,7 @@ const createFilter: CreateFilter = function createFilter(include?, exclude?, opt
     if (typeof id !== 'string') return false;
     if (/\0/.test(id)) return false;
 
-    const pathId = id.split(sep).join('/');
+    const pathId = normalizePath(id);
 
     for (let i = 0; i < excludeMatchers.length; ++i) {
       const matcher = excludeMatchers[i];
diff --git a/packages/pluginutils/src/index.ts b/packages/pluginutils/src/index.ts
index ac39803fe..61df2bcf9 100644
--- a/packages/pluginutils/src/index.ts
+++ b/packages/pluginutils/src/index.ts
@@ -4,6 +4,7 @@ import createFilter from './createFilter';
 import dataToEsm from './dataToEsm';
 import extractAssignedNames from './extractAssignedNames';
 import makeLegalIdentifier from './makeLegalIdentifier';
+import normalizePath from './normalizePath';
 
 export {
   addExtension,
@@ -11,7 +12,8 @@ export {
   createFilter,
   dataToEsm,
   extractAssignedNames,
-  makeLegalIdentifier
+  makeLegalIdentifier,
+  normalizePath
 };
 
 // TODO: remove this in next major
@@ -21,5 +23,6 @@ export default {
   createFilter,
   dataToEsm,
   extractAssignedNames,
-  makeLegalIdentifier
+  makeLegalIdentifier,
+  normalizePath
 };
diff --git a/packages/pluginutils/src/normalizePath.ts b/packages/pluginutils/src/normalizePath.ts
new file mode 100644
index 000000000..0b0b7f6a7
--- /dev/null
+++ b/packages/pluginutils/src/normalizePath.ts
@@ -0,0 +1,9 @@
+import { win32, posix } from 'path';
+
+import { NormalizePath } from '../types';
+
+const normalizePath: NormalizePath = function(filename: string) {
+  return filename.split(win32.sep).join(posix.sep);
+};
+
+export { normalizePath as default };
diff --git a/packages/pluginutils/test/createFilter.ts b/packages/pluginutils/test/createFilter.ts
index 7a68ca999..ac878e9b3 100755
--- a/packages/pluginutils/test/createFilter.ts
+++ b/packages/pluginutils/test/createFilter.ts
@@ -1,8 +1,10 @@
-import { resolve } from 'path';
+import { resolve as rawResolve } from 'path';
 
 import test from 'ava';
 
-import { createFilter } from '../';
+import { createFilter, normalizePath } from '../';
+
+const resolve = (...parts: string[]) => normalizePath(rawResolve(...parts));
 
 test.beforeEach(() => process.chdir(__dirname));
 
diff --git a/packages/pluginutils/test/normalizePath.ts b/packages/pluginutils/test/normalizePath.ts
new file mode 100644
index 000000000..06c9914c5
--- /dev/null
+++ b/packages/pluginutils/test/normalizePath.ts
@@ -0,0 +1,17 @@
+import test from 'ava';
+
+import { normalizePath } from '../';
+
+test('replaces \\ with /', (t) => {
+  t.is(normalizePath('foo\\bar'), 'foo/bar');
+  t.is(normalizePath('foo\\bar\\baz'), 'foo/bar/baz');
+});
+
+test('ignores forward slash', (t) => {
+  t.is(normalizePath('foo/bar'), 'foo/bar');
+  t.is(normalizePath('foo/bar\\baz'), 'foo/bar/baz');
+});
+
+test('handles empty string', (t) => {
+  t.is(normalizePath(''), '');
+});
diff --git a/packages/pluginutils/types/index.d.ts b/packages/pluginutils/types/index.d.ts
index 33d40e509..39f7ab87a 100755
--- a/packages/pluginutils/types/index.d.ts
+++ b/packages/pluginutils/types/index.d.ts
@@ -68,11 +68,17 @@ export function extractAssignedNames(param: BaseNode): string[];
  */
 export function makeLegalIdentifier(str: string): string;
 
+/**
+ * Converts path separators to forward slash.
+ */
+export function normalizePath(filename: string): string;
+
 export type AddExtension = typeof addExtension;
 export type AttachScopes = typeof attachScopes;
 export type CreateFilter = typeof createFilter;
 export type ExtractAssignedNames = typeof extractAssignedNames;
 export type MakeLegalIdentifier = typeof makeLegalIdentifier;
+export type NormalizePath = typeof normalizePath;
 export type DataToEsm = typeof dataToEsm;
 
 declare const defaultExport: {
@@ -82,5 +88,6 @@ declare const defaultExport: {
   dataToEsm: DataToEsm;
   extractAssignedNames: ExtractAssignedNames;
   makeLegalIdentifier: MakeLegalIdentifier;
+  normalizePath: NormalizePath;
 };
 export default defaultExport;