diff --git a/doc/api/esm.md b/doc/api/esm.md
index e5c957d03ce6a0..8a3d5bdecbdb2e 100644
--- a/doc/api/esm.md
+++ b/doc/api/esm.md
@@ -1196,16 +1196,27 @@ export async function resolve(specifier, context, defaultResolve) {
 > signature may change. Do not rely on the API described below.
 
 The `getFormat` hook provides a way to define a custom method of determining how
-a URL should be interpreted. This can be one of the following:
+a URL should be interpreted. The `format` returned also affects what the
+acceptable forms of source values are for a module when parsing. This can be one
+of the following:
 
-| `format` | Description |
-| --- | --- |
-| `'builtin'` | Load a Node.js builtin module |
-| `'commonjs'` | Load a Node.js CommonJS module |
-| `'dynamic'` | Use a [dynamic instantiate hook][] |
-| `'json'` | Load a JSON file |
-| `'module'` | Load a standard JavaScript module (ES module) |
-| `'wasm'` | Load a WebAssembly module |
+| `format` | Description | Acceptable Types For `source` Returned by `getSource` or `transformSource` |
+| --- | --- | --- |
+| `'builtin'` | Load a Node.js builtin module | Not applicable |
+| `'commonjs'` | Load a Node.js CommonJS module | Not applicable |
+| `'dynamic'` | Use a [dynamic instantiate hook][] | Not applicable |
+| `'json'` | Load a JSON file | { [ArrayBuffer][], [string][], [TypedArray][] } |
+| `'module'` | Load an ES module | { [ArrayBuffer][], [string][], [TypedArray][] } |
+| `'wasm'` | Load a WebAssembly module | { [ArrayBuffer][], [string][], [TypedArray][] } |
+
+Note: These types all correspond to classes defined in ECMAScript.
+
+* The specific [ArrayBuffer][] object is a [SharedArrayBuffer][].
+* The specific [string][] object is not the class constructor, but an instance.
+* The specific [TypedArray][] object is a [Uint8Array][].
+
+Note: If the source value of a text-based format (i.e., `'json'`, `'module'`) is
+not a string, it will be converted to a string using [`util.TextDecoder`][].
 
 ```js
 /**
@@ -1842,6 +1853,12 @@ success!
 [`module.createRequire()`]: modules.html#modules_module_createrequire_filename
 [`module.syncBuiltinESMExports()`]: modules.html#modules_module_syncbuiltinesmexports
 [`transformSource` hook]: #esm_code_transformsource_code_hook
+[ArrayBuffer]: http://www.ecma-international.org/ecma-262/6.0/#sec-arraybuffer-constructor
+[SharedArrayBuffer]: https://tc39.es/ecma262/#sec-sharedarraybuffer-constructor
+[string]: http://www.ecma-international.org/ecma-262/6.0/#sec-string-constructor
+[TypedArray]: http://www.ecma-international.org/ecma-262/6.0/#sec-typedarray-objects
+[Uint8Array]: http://www.ecma-international.org/ecma-262/6.0/#sec-uint8array
+[`util.TextDecoder`]: util.html#util_class_util_textdecoder
 [dynamic instantiate hook]: #esm_code_dynamicinstantiate_code_hook
 [import an ES or CommonJS module for its side effects only]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#Import_a_module_for_its_side_effects_only
 [special scheme]: https://url.spec.whatwg.org/#special-scheme
diff --git a/lib/internal/modules/esm/translators.js b/lib/internal/modules/esm/translators.js
index a7252c1c99f954..f314ba96b3476c 100644
--- a/lib/internal/modules/esm/translators.js
+++ b/lib/internal/modules/esm/translators.js
@@ -11,6 +11,12 @@ const {
   StringPrototypeReplace,
 } = primordials;
 
+let _TYPES = null;
+function lazyTypes() {
+  if (_TYPES !== null) return _TYPES;
+  return _TYPES = require('internal/util/types');
+}
+
 const {
   stripBOM,
   loadNativeModule
@@ -26,7 +32,10 @@ const createDynamicModule = require(
 const { fileURLToPath, URL } = require('url');
 const { debuglog } = require('internal/util/debuglog');
 const { emitExperimentalWarning } = require('internal/util');
-const { ERR_UNKNOWN_BUILTIN_MODULE } = require('internal/errors').codes;
+const {
+  ERR_UNKNOWN_BUILTIN_MODULE,
+  ERR_INVALID_RETURN_PROPERTY_VALUE
+} = require('internal/errors').codes;
 const { maybeCacheSourceMap } = require('internal/source_map/source_map_cache');
 const moduleWrap = internalBinding('module_wrap');
 const { ModuleWrap } = moduleWrap;
@@ -39,6 +48,30 @@ const debug = debuglog('esm');
 const translators = new SafeMap();
 exports.translators = translators;
 
+let DECODER = null;
+function assertBufferSource(body, allowString, hookName) {
+  if (allowString && typeof body === 'string') {
+    return;
+  }
+  const { isArrayBufferView, isAnyArrayBuffer } = lazyTypes();
+  if (isArrayBufferView(body) || isAnyArrayBuffer(body)) {
+    return;
+  }
+  throw new ERR_INVALID_RETURN_PROPERTY_VALUE(
+    `${allowString ? 'string, ' : ''}array buffer, or typed array`,
+    hookName,
+    'source',
+    body
+  );
+}
+
+function stringify(body) {
+  if (typeof body === 'string') return body;
+  assertBufferSource(body, false, 'transformSource');
+  DECODER = DECODER === null ? new TextDecoder() : DECODER;
+  return DECODER.decode(body);
+}
+
 function errPath(url) {
   const parsed = new URL(url);
   if (parsed.protocol === 'file:') {
@@ -80,9 +113,10 @@ function initializeImportMeta(meta, { url }) {
 translators.set('module', async function moduleStrategy(url) {
   let { source } = await this._getSource(
     url, { format: 'module' }, defaultGetSource);
-  source = `${source}`;
+  assertBufferSource(source, true, 'getSource');
   ({ source } = await this._transformSource(
     source, { url, format: 'module' }, defaultTransformSource));
+  source = stringify(source);
   maybeCacheSourceMap(url, source);
   debug(`Translating StandardModule ${url}`);
   const module = new ModuleWrap(url, undefined, source, 0, 0);
@@ -157,9 +191,10 @@ translators.set('json', async function jsonStrategy(url) {
   }
   let { source } = await this._getSource(
     url, { format: 'json' }, defaultGetSource);
-  source = `${source}`;
+  assertBufferSource(source, true, 'getSource');
   ({ source } = await this._transformSource(
     source, { url, format: 'json' }, defaultTransformSource));
+  source = stringify(source);
   if (pathname) {
     // A require call could have been called on the same file during loading and
     // that resolves synchronously. To make sure we always return the identical
@@ -200,8 +235,10 @@ translators.set('wasm', async function(url) {
   emitExperimentalWarning('Importing Web Assembly modules');
   let { source } = await this._getSource(
     url, { format: 'wasm' }, defaultGetSource);
+  assertBufferSource(source, false, 'getSource');
   ({ source } = await this._transformSource(
     source, { url, format: 'wasm' }, defaultTransformSource));
+  assertBufferSource(source, false, 'transformSource');
   debug(`Translating WASMModule ${url}`);
   let compiled;
   try {
diff --git a/test/es-module/test-esm-loader-stringify-text.mjs b/test/es-module/test-esm-loader-stringify-text.mjs
new file mode 100644
index 00000000000000..ed9bd5069ebf1d
--- /dev/null
+++ b/test/es-module/test-esm-loader-stringify-text.mjs
@@ -0,0 +1,50 @@
+// Flags: --experimental-loader ./test/fixtures/es-module-loaders/string-sources.mjs
+import { mustCall, mustNotCall } from '../common/index.mjs';
+import assert from 'assert';
+
+import('test:Array').then(
+  mustNotCall('Should not accept Arrays'),
+  mustCall((e) => {
+    assert.strictEqual(e.code, 'ERR_INVALID_RETURN_PROPERTY_VALUE');
+  })
+);
+import('test:ArrayBuffer').then(
+  mustCall(),
+  mustNotCall('Should accept ArrayBuffers'),
+);
+import('test:null').then(
+  mustNotCall('Should not accept null'),
+  mustCall((e) => {
+    assert.strictEqual(e.code, 'ERR_INVALID_RETURN_PROPERTY_VALUE');
+  })
+);
+import('test:Object').then(
+  mustNotCall('Should not stringify or valueOf Objects'),
+  mustCall((e) => {
+    assert.strictEqual(e.code, 'ERR_INVALID_RETURN_PROPERTY_VALUE');
+  })
+);
+import('test:SharedArrayBuffer').then(
+  mustCall(),
+  mustNotCall('Should accept SharedArrayBuffers'),
+);
+import('test:string').then(
+  mustCall(),
+  mustNotCall('Should accept strings'),
+);
+import('test:String').then(
+  mustNotCall('Should not accept wrapper Strings'),
+  mustCall((e) => {
+    assert.strictEqual(e.code, 'ERR_INVALID_RETURN_PROPERTY_VALUE');
+  })
+);
+import('test:Uint8Array').then(
+  mustCall(),
+  mustNotCall('Should accept Uint8Arrays'),
+);
+import('test:undefined').then(
+  mustNotCall('Should not accept undefined'),
+  mustCall((e) => {
+    assert.strictEqual(e.code, 'ERR_INVALID_RETURN_PROPERTY_VALUE');
+  })
+);
diff --git a/test/fixtures/es-module-loaders/string-sources.mjs b/test/fixtures/es-module-loaders/string-sources.mjs
new file mode 100644
index 00000000000000..3133231208ce1e
--- /dev/null
+++ b/test/fixtures/es-module-loaders/string-sources.mjs
@@ -0,0 +1,30 @@
+const SOURCES = {
+  __proto__: null,
+  'test:Array': ['1', '2'], // both `1,2` and `12` are valid ESM
+  'test:ArrayBuffer': new ArrayBuffer(0),
+  'test:null': null,
+  'test:Object': {},
+  'test:SharedArrayBuffer': new SharedArrayBuffer(0),
+  'test:string': '',
+  'test:String': new String(''),
+  'test:Uint8Array': new Uint8Array(0),
+  'test:undefined': undefined,
+}
+export function resolve(specifier, context, defaultFn) {
+  if (specifier.startsWith('test:')) {
+    return { url: specifier };
+  }
+  return defaultFn(specifier, context);
+}
+export function getFormat(href, context, defaultFn) {
+  if (href.startsWith('test:')) {
+    return { format: 'module' };
+  }
+  return defaultFn(href, context);
+}
+export function getSource(href, context, defaultFn) {
+  if (href.startsWith('test:')) {
+    return { source: SOURCES[href] };
+  }
+  return defaultFn(href, context);
+}
diff --git a/test/fixtures/es-module-loaders/transform-source.mjs b/test/fixtures/es-module-loaders/transform-source.mjs
index ab147c34cb34fd..25d983b64e62ca 100644
--- a/test/fixtures/es-module-loaders/transform-source.mjs
+++ b/test/fixtures/es-module-loaders/transform-source.mjs
@@ -1,6 +1,9 @@
 export async function transformSource(
   source, { url, format }, defaultTransformSource) {
-  if (source && source.replace) {
+  if (format === 'module') {
+    if (typeof source !== 'string') {
+      source = new TextDecoder().decode(source);
+    }
     return {
       source: source.replace(`'A message';`, `'A message'.toUpperCase();`)
     };