diff --git a/node/index.d.ts b/node/index.d.ts
index e1a43102..a9f2a7f2 100644
--- a/node/index.d.ts
+++ b/node/index.d.ts
@@ -197,3 +197,18 @@ export declare function browserslistToTargets(browserslist: string[]): Targets;
  * Bundles a CSS file and its dependencies, inlining @import rules.
  */
 export declare function bundle(options: BundleOptions): TransformResult;
+
+export interface AsyncBundleOptions extends BundleOptions {
+  resolver?: Resolver;
+}
+
+export interface Resolver {
+  read?: (file: string) => string | Promise<string>;
+  resolve?: (specifier: string, originatingFile: string) => string | Promise<string>;
+}
+
+/**
+ * Bundles a CSS file and its dependencies asynchronously, inlining @import rules.
+ * Also supports custom resolvers.
+ */
+export declare function bundleAsync(options: AsyncBundleOptions): Promise<TransformResult>;
diff --git a/node/index.js b/node/index.js
index ba7fe4ee..508de4e9 100644
--- a/node/index.js
+++ b/node/index.js
@@ -23,3 +23,48 @@ if (process.env.CSS_TRANSFORMER_WASM) {
 }
 
 module.exports.browserslistToTargets = require('./browserslistToTargets');
+
+// Include a small JS-shim for `bundleAsync()` to convert make `resolver` more ergonomic.
+const {bundleAsync} = module.exports;
+module.exports.bundleAsync = (opts, ...rest) => {
+  return bundleAsync({
+    ...opts,
+    resolver: opts.resolver && {
+      ...opts.resolver,
+      read: opts.resolver.read && normalizeJsCallback(opts.resolver.read.bind(opts.resolver)),
+      resolve: opts.resolver.resolve && normalizeJsCallback(opts.resolver.resolve.bind(opts.resolver)),
+    },
+  }, ...rest);
+};
+
+// `napi-rs` ignores JS function return values, so any results must be passed back to
+// the Rust side via a callback rather than a returned value or `Promise`. This
+// callback also follows NodeJS conventions (`callback(err, result)`). Managing the
+// error and callback are annoying for users, so this converts a typical JS function
+// which returns its result in a `Promise` into a N-API-compatible function which
+// accepts and propagates its results to a callback.
+function normalizeJsCallback(func) {
+  return (...args) => {
+    // Splice out `[...args, callback]`.
+    const funcArgs = args.slice(0, -1);
+    const callback = args[args.length - 1];
+
+    // Invoke the inner function, normalize to a `Promise`, and then invoke the callback
+    // function with Node conventions of `callback(err, result)`.
+    toPromise(() => func(...funcArgs)).then(
+      (result) => callback(null, result),
+      (err) => callback(err, null),
+    );
+  };
+}
+
+// Converts the given function execution to return a `Promise` instead of returning or
+// erroring synchronously. This is different from `Promise.resolve(func())` in that a
+// synchronous `throw` statement is converted to a `Promise` rejection.
+function toPromise(func) {
+  try {
+    return Promise.resolve(func());
+  } catch (err) {
+    return Promise.reject(err);
+  }
+}
diff --git a/test-bundle.mjs b/test-bundle.mjs
new file mode 100644
index 00000000..eb1ad310
--- /dev/null
+++ b/test-bundle.mjs
@@ -0,0 +1,262 @@
+import path from 'path';
+import css from './node/index.js';
+
+(async function testResolver() {
+    const inMemoryFs = new Map(Object.entries({
+        'foo.css': `
+@import 'root:bar.css';
+
+.foo { color: red; }
+        `.trim(),
+
+        'bar.css': `
+@import 'root:hello/world.css';
+
+.bar { color: green; }
+        `.trim(),
+
+        'hello/world.css': `
+.baz { color: blue; }
+        `.trim(),
+    }));
+
+    const { code: buffer } = await css.bundleAsync({
+        filename: 'foo.css',
+        resolver: {
+            read(file) {
+                const result = inMemoryFs.get(path.normalize(file));
+                if (!result) throw new Error(`Could not find ${file} in ${
+                    Array.from(inMemoryFs.keys()).join(', ')}.`);
+                return result;
+            },
+
+            resolve(specifier) {
+                return specifier.slice('root:'.length);
+            },
+        },
+    });
+    const code = buffer.toString('utf-8').trim();
+
+    const expected = `
+.baz {
+  color: #00f;
+}
+
+.bar {
+  color: green;
+}
+
+.foo {
+  color: red;
+}
+    `.trim();
+    if (code !== expected) throw new Error(`\`testResolver()\` failed. Expected:\n${expected}\n\nGot:\n${code}`);
+})();
+
+(async function testOnlyCustomRead() {
+    const inMemoryFs = new Map(Object.entries({
+        'foo.css': `
+@import 'hello/world.css';
+
+.foo { color: red; }
+        `.trim(),
+
+        'hello/world.css': `
+@import '../bar.css';
+
+.bar { color: green; }
+        `.trim(),
+
+        'bar.css': `
+.baz { color: blue; }
+        `.trim(),
+    }));
+
+    const { code: buffer } = await css.bundleAsync({
+        filename: 'foo.css',
+        resolver: {
+            read(file) {
+                const result = inMemoryFs.get(path.normalize(file));
+                if (!result) throw new Error(`Could not find ${file} in ${
+                    Array.from(inMemoryFs.keys()).join(', ')}.`);
+                return result;
+            },
+        },
+    });
+    const code = buffer.toString('utf-8').trim();
+
+    const expected = `
+.baz {
+  color: #00f;
+}
+
+.bar {
+  color: green;
+}
+
+.foo {
+  color: red;
+}
+    `.trim();
+    if (code !== expected) throw new Error(`\`testOnlyCustomRead()\` failed. Expected:\n${expected}\n\nGot:\n${code}`);
+})();
+
+(async function testOnlyCustomResolve() {
+    const root = path.join('tests', 'testdata', 'css');
+    const { code: buffer } = await css.bundleAsync({
+        filename: path.join(root, 'foo.css'),
+        resolver: {
+            resolve(specifier) {
+                // Strip `root:` prefix off specifier and resolve it as an absolute path
+                // in the test data root.
+                return path.join(root, specifier.slice('root:'.length));
+            },
+        },
+    });
+    const code = buffer.toString('utf-8').trim();
+
+    const expected = `
+.baz {
+  color: #00f;
+}
+
+.bar {
+  color: green;
+}
+
+.foo {
+  color: red;
+}
+    `.trim();
+    if (code !== expected) throw new Error(`\`testOnlyCustomResolve()\` failed. Expected:\n${expected}\n\nGot:\n${code}`);
+})();
+
+(async function testReadThrow() {
+    let error = undefined;
+    try {
+        await css.bundleAsync({
+            filename: 'foo.css',
+            resolver: {
+                read(file) {
+                    throw new Error(`Oh noes! Failed to read \`${file}\`.`);
+                }
+            },
+        });
+    } catch (err) {
+        error = err;
+    }
+
+    if (!error) throw new Error(`\`testReadThrow()\` failed. Expected \`bundleAsync()\` to throw, but it did not.`);
+    if (!error.message.includes(`\`read()\` threw error:`) || !error.message.includes(`Oh noes! Failed to read \`foo.css\`.`)) {
+        throw new Error(`\`testReadThrow()\` failed. Expected \`bundleAsync()\` to throw a specific error message, but it threw a different error:\n${error.message}`);
+    }
+})();
+
+(async function testResolveThrow() {
+    let error = undefined;
+    try {
+        await css.bundleAsync({
+            filename: 'tests/testdata/css/foo.css',
+            resolver: {
+                resolve(specifier, originatingFile) {
+                    throw new Error(`Oh noes! Failed to resolve \`${specifier}\` from \`${
+                        originatingFile}\`.`);
+                }
+            },
+        });
+    } catch (err) {
+        error = err;
+    }
+
+    if (!error) throw new Error(`\`testResolveThrow()\` failed. Expected \`bundleAsync()\` to throw, but it did not.`);
+    if (!error.message.includes(`\`resolve()\` threw error:`) || !error.message.includes(`Oh noes! Failed to resolve \`root:hello/world.css\` from \`tests/testdata/css/foo.css\`.`)) {
+        throw new Error(`\`testResolveThrow()\` failed. Expected \`bundleAsync()\` to throw a specific error message, but it threw a different error:\n${error.message}`);
+    }
+})();
+
+(async function testReadReturnNonString() {
+    let error = undefined;
+    try {
+        await css.bundleAsync({
+            filename: 'foo.css',
+            resolver: {
+                read() {
+                    return 1234; // Returns a non-string value.
+                }
+            },
+        });
+    } catch (err) {
+        error = err;
+    }
+
+    if (!error) throw new Error(`\`testReadReturnNonString()\` failed. Expected \`bundleAsync()\` to throw, but it did not.`);
+    if (!error.message.includes(`Expected \`read()\` to return a value of type \`String\`, but it returned a value of type \`Number\` instead.`)) {
+        throw new Error(`\`testReadReturnNonString()\` failed. Expected \`bundleAsync()\` to throw a specific error message, but it threw a different error:\n${error.message}`);
+    }
+})();
+
+(async function testResolveReturnNonString() {
+    let error = undefined;
+    try {
+        await css.bundleAsync({
+            filename: 'tests/testdata/css/foo.css',
+            resolver: {
+                resolve() {
+                    return 1234; // Returns a non-string value.
+                }
+            },
+        });
+    } catch (err) {
+        error = err;
+    }
+
+    if (!error) throw new Error(`\`testResolveReturnNonString()\` failed. Expected \`bundleAsync()\` to throw, but it did not.`);
+    if (!error.message.includes(`Expected \`resolve()\` to return a value of type \`String\`, but it returned a value of type \`Number\` instead.`)) {
+        throw new Error(`\`testResolveReturnNonString()\` failed. Expected \`bundleAsync()\` to throw a specific error message, but it threw a different error:\n${error.message}`);
+    }
+})();
+
+(async function testThis() {
+    const inMemoryFs = new Map(Object.entries({
+        'foo.css': `
+@import './bar.css';
+
+.foo { color: red; }
+        `.trim(),
+
+        'bar.css': `
+@import './hello/world.css';
+
+.bar { color: green; }
+        `.trim(),
+
+        'hello/world.css': `
+.baz { color: blue; }
+        `.trim(),
+    }));
+
+    let readThis = undefined;
+    let resolveThis = undefined;
+    const resolver = {
+        read: function (file) {
+            readThis = this;
+            const result = inMemoryFs.get(path.normalize(file));
+            if (!result) throw new Error(`Could not find ${file} in ${
+                Array.from(inMemoryFs.keys()).join(', ')}.`);
+            return result;
+        },
+        resolve: function (specifier, originatingFile) {
+            resolveThis = this;
+            return path.join(originatingFile, '..', specifier);
+        },
+    };
+    await css.bundleAsync({
+        filename: 'foo.css',
+        resolver,
+    });
+
+    if (readThis !== resolver) throw new Error(`\`testThis()\` failed. Expected \`read()\` to be called with the \`resolver\` as \`this\`, but instead it was called with:\n${readThis}`);
+    if (resolveThis !== resolver) throw new Error(`\`testThis()\` failed. Expected \`resolve()\` to be called with the \`resolver\` as \`this\`, but instead it was called with:\n${resolveThis}`);
+})();
+
+console.log('PASSED!');
diff --git a/tests/testdata/css/baz.css b/tests/testdata/css/baz.css
new file mode 100644
index 00000000..ccb3274e
--- /dev/null
+++ b/tests/testdata/css/baz.css
@@ -0,0 +1 @@
+.baz { color: blue; }
diff --git a/tests/testdata/css/foo.css b/tests/testdata/css/foo.css
new file mode 100644
index 00000000..95099612
--- /dev/null
+++ b/tests/testdata/css/foo.css
@@ -0,0 +1,3 @@
+@import 'root:hello/world.css';
+
+.foo { color: red; }
diff --git a/tests/testdata/css/hello/world.css b/tests/testdata/css/hello/world.css
new file mode 100644
index 00000000..bc1763bb
--- /dev/null
+++ b/tests/testdata/css/hello/world.css
@@ -0,0 +1,3 @@
+@import 'root:baz.css';
+
+.bar { color: green; }