diff --git a/doc/api/wasi.md b/doc/api/wasi.md
index 46673af9e7fb34..9fe004ea599cfc 100644
--- a/doc/api/wasi.md
+++ b/doc/api/wasi.md
@@ -132,6 +132,23 @@ Attempt to begin execution of `instance` as a WASI command by invoking its
 
 If `start()` is called more than once, an exception is thrown.
 
+### `wasi.initialize(instance)`
+<!-- YAML
+added:
+ - REPLACEME
+-->
+
+* `instance` {WebAssembly.Instance}
+
+Attempt to initialize `instance` as a WASI reactor by invoking its
+`_initialize()` export, if it is present. If `instance` contains a `_start()`
+export, then an exception is thrown.
+
+`initialize()` requires that `instance` exports a [`WebAssembly.Memory`][] named
+`memory`. If `instance` does not have a `memory` export an exception is thrown.
+
+If `initialize()` is called more than once, an exception is thrown.
+
 ### `wasi.wasiImport`
 <!-- YAML
 added:
diff --git a/lib/wasi.js b/lib/wasi.js
index 97268f1b9d2fbc..a764a1745ab0a9 100644
--- a/lib/wasi.js
+++ b/lib/wasi.js
@@ -20,13 +20,36 @@ const {
   validateObject,
 } = require('internal/validators');
 const { WASI: _WASI } = internalBinding('wasi');
-const kExitCode = Symbol('exitCode');
-const kSetMemory = Symbol('setMemory');
-const kStarted = Symbol('started');
+const kExitCode = Symbol('kExitCode');
+const kSetMemory = Symbol('kSetMemory');
+const kStarted = Symbol('kStarted');
+const kInstance = Symbol('kInstance');
 
 emitExperimentalWarning('WASI');
 
 
+function setupInstance(self, instance) {
+  validateObject(instance, 'instance');
+  validateObject(instance.exports, 'instance.exports');
+
+  // WASI::_SetMemory() in src/node_wasi.cc only expects that |memory| is
+  // an object. It will try to look up the .buffer property when needed
+  // and fail with UVWASI_EINVAL when the property is missing or is not
+  // an ArrayBuffer. Long story short, we don't need much validation here
+  // but we type-check anyway because it helps catch bugs in the user's
+  // code early.
+  validateObject(instance.exports.memory, 'instance.exports.memory');
+  if (!isArrayBuffer(instance.exports.memory.buffer)) {
+    throw new ERR_INVALID_ARG_TYPE(
+      'instance.exports.memory.buffer',
+      ['WebAssembly.Memory'],
+      instance.exports.memory.buffer);
+  }
+
+  self[kInstance] = instance;
+  self[kSetMemory](instance.exports.memory);
+}
+
 class WASI {
   constructor(options = {}) {
     validateObject(options, 'options');
@@ -75,58 +98,63 @@ class WASI {
     this.wasiImport = wrap;
     this[kStarted] = false;
     this[kExitCode] = 0;
+    this[kInstance] = undefined;
   }
 
+  // Must not export _initialize, must export _start
   start(instance) {
-    validateObject(instance, 'instance');
-
-    const exports = instance.exports;
+    if (this[kStarted]) {
+      throw new ERR_WASI_ALREADY_STARTED();
+    }
+    this[kStarted] = true;
 
-    validateObject(exports, 'instance.exports');
+    setupInstance(this, instance);
 
-    const { _initialize, _start, memory } = exports;
+    const { _start, _initialize } = this[kInstance].exports;
 
     if (typeof _start !== 'function') {
       throw new ERR_INVALID_ARG_TYPE(
         'instance.exports._start', 'function', _start);
     }
-
     if (_initialize !== undefined) {
       throw new ERR_INVALID_ARG_TYPE(
         'instance.exports._initialize', 'undefined', _initialize);
     }
 
-    // WASI::_SetMemory() in src/node_wasi.cc only expects that |memory| is
-    // an object. It will try to look up the .buffer property when needed
-    // and fail with UVWASI_EINVAL when the property is missing or is not
-    // an ArrayBuffer. Long story short, we don't need much validation here
-    // but we type-check anyway because it helps catch bugs in the user's
-    // code early.
-    validateObject(memory, 'instance.exports.memory');
-
-    if (!isArrayBuffer(memory.buffer)) {
-      throw new ERR_INVALID_ARG_TYPE(
-        'instance.exports.memory.buffer',
-        ['WebAssembly.Memory'],
-        memory.buffer);
+    try {
+      _start();
+    } catch (err) {
+      if (err !== kExitCode) {
+        throw err;
+      }
     }
 
+    return this[kExitCode];
+  }
+
+  // Must not export _start, may optionally export _initialize
+  initialize(instance) {
     if (this[kStarted]) {
       throw new ERR_WASI_ALREADY_STARTED();
     }
-
     this[kStarted] = true;
-    this[kSetMemory](memory);
 
-    try {
-      exports._start();
-    } catch (err) {
-      if (err !== kExitCode) {
-        throw err;
-      }
+    setupInstance(this, instance);
+
+    const { _start, _initialize } = this[kInstance].exports;
+
+    if (typeof _initialize !== 'function' && _initialize !== undefined) {
+      throw new ERR_INVALID_ARG_TYPE(
+        'instance.exports._initialize', 'function', _initialize);
+    }
+    if (_start !== undefined) {
+      throw new ERR_INVALID_ARG_TYPE(
+        'instance.exports._start', 'undefined', _initialize);
     }
 
-    return this[kExitCode];
+    if (_initialize !== undefined) {
+      _initialize();
+    }
   }
 }
 
diff --git a/test/wasi/test-wasi-initialize-validation.js b/test/wasi/test-wasi-initialize-validation.js
new file mode 100644
index 00000000000000..79b0bd8485a483
--- /dev/null
+++ b/test/wasi/test-wasi-initialize-validation.js
@@ -0,0 +1,195 @@
+// Flags: --experimental-wasi-unstable-preview1
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const vm = require('vm');
+const { WASI } = require('wasi');
+
+const fixtures = require('../common/fixtures');
+const bufferSource = fixtures.readSync('simple.wasm');
+
+(async () => {
+  {
+    // Verify that a WebAssembly.Instance is passed in.
+    const wasi = new WASI();
+
+    assert.throws(
+      () => { wasi.initialize(); },
+      {
+        code: 'ERR_INVALID_ARG_TYPE',
+        message: /"instance" argument must be of type object/
+      }
+    );
+  }
+
+  {
+    // Verify that the passed instance has an exports objects.
+    const wasi = new WASI({});
+    const wasm = await WebAssembly.compile(bufferSource);
+    const instance = await WebAssembly.instantiate(wasm);
+
+    Object.defineProperty(instance, 'exports', { get() { return null; } });
+    assert.throws(
+      () => { wasi.initialize(instance); },
+      {
+        code: 'ERR_INVALID_ARG_TYPE',
+        message: /"instance\.exports" property must be of type object/
+      }
+    );
+  }
+
+  {
+    // Verify that a _initialize() export was passed.
+    const wasi = new WASI({});
+    const wasm = await WebAssembly.compile(bufferSource);
+    const instance = await WebAssembly.instantiate(wasm);
+
+    Object.defineProperty(instance, 'exports', {
+      get() {
+        return { _initialize: 5, memory: new Uint8Array() };
+      },
+    });
+    assert.throws(
+      () => { wasi.initialize(instance); },
+      {
+        code: 'ERR_INVALID_ARG_TYPE',
+        message: /"instance\.exports\._initialize" property must be of type function/
+      }
+    );
+  }
+
+  {
+    // Verify that a _start export was not passed.
+    const wasi = new WASI({});
+    const wasm = await WebAssembly.compile(bufferSource);
+    const instance = await WebAssembly.instantiate(wasm);
+
+    Object.defineProperty(instance, 'exports', {
+      get() {
+        return {
+          _start() {},
+          _initialize() {},
+          memory: new Uint8Array(),
+        };
+      }
+    });
+    assert.throws(
+      () => { wasi.initialize(instance); },
+      {
+        code: 'ERR_INVALID_ARG_TYPE',
+        message: /"instance\.exports\._start" property must be undefined/
+      }
+    );
+  }
+
+  {
+    // Verify that a memory export was passed.
+    const wasi = new WASI({});
+    const wasm = await WebAssembly.compile(bufferSource);
+    const instance = await WebAssembly.instantiate(wasm);
+
+    Object.defineProperty(instance, 'exports', {
+      get() { return { _initialize() {} }; }
+    });
+    assert.throws(
+      () => { wasi.initialize(instance); },
+      {
+        code: 'ERR_INVALID_ARG_TYPE',
+        message: /"instance\.exports\.memory" property must be of type object/
+      }
+    );
+  }
+
+  {
+    // Verify that a non-ArrayBuffer memory.buffer is rejected.
+    const wasi = new WASI({});
+    const wasm = await WebAssembly.compile(bufferSource);
+    const instance = await WebAssembly.instantiate(wasm);
+
+    Object.defineProperty(instance, 'exports', {
+      get() {
+        return {
+          _initialize() {},
+          memory: {},
+        };
+      }
+    });
+    // The error message is a little white lie because any object
+    // with a .buffer property of type ArrayBuffer is accepted,
+    // but 99% of the time a WebAssembly.Memory object is used.
+    assert.throws(
+      () => { wasi.initialize(instance); },
+      {
+        code: 'ERR_INVALID_ARG_TYPE',
+        message: /"instance\.exports\.memory\.buffer" property must be an WebAssembly\.Memory/
+      }
+    );
+  }
+
+  {
+    // Verify that an argument that duck-types as a WebAssembly.Instance
+    // is accepted.
+    const wasi = new WASI({});
+    const wasm = await WebAssembly.compile(bufferSource);
+    const instance = await WebAssembly.instantiate(wasm);
+
+    Object.defineProperty(instance, 'exports', {
+      get() {
+        return {
+          _initialize() {},
+          memory: { buffer: new ArrayBuffer(0) },
+        };
+      }
+    });
+    wasi.initialize(instance);
+  }
+
+  {
+    // Verify that a WebAssembly.Instance from another VM context is accepted.
+    const wasi = new WASI({});
+    const instance = await vm.runInNewContext(`
+      (async () => {
+        const wasm = await WebAssembly.compile(bufferSource);
+        const instance = await WebAssembly.instantiate(wasm);
+
+        Object.defineProperty(instance, 'exports', {
+          get() {
+            return {
+              _initialize() {},
+              memory: new WebAssembly.Memory({ initial: 1 })
+            };
+          }
+        });
+
+        return instance;
+      })()
+    `, { bufferSource });
+
+    wasi.initialize(instance);
+  }
+
+  {
+    // Verify that initialize() can only be called once.
+    const wasi = new WASI({});
+    const wasm = await WebAssembly.compile(bufferSource);
+    const instance = await WebAssembly.instantiate(wasm);
+
+    Object.defineProperty(instance, 'exports', {
+      get() {
+        return {
+          _initialize() {},
+          memory: new WebAssembly.Memory({ initial: 1 })
+        };
+      }
+    });
+    wasi.initialize(instance);
+    assert.throws(
+      () => { wasi.initialize(instance); },
+      {
+        code: 'ERR_WASI_ALREADY_STARTED',
+        message: /^WASI instance has already started$/
+      }
+    );
+  }
+})().then(common.mustCall());
diff --git a/test/wasi/test-wasi-start-validation.js b/test/wasi/test-wasi-start-validation.js
index 3134514d704595..5c6a1ede5d4fd7 100644
--- a/test/wasi/test-wasi-start-validation.js
+++ b/test/wasi/test-wasi-start-validation.js
@@ -45,7 +45,11 @@ const bufferSource = fixtures.readSync('simple.wasm');
     const wasm = await WebAssembly.compile(bufferSource);
     const instance = await WebAssembly.instantiate(wasm);
 
-    Object.defineProperty(instance, 'exports', { get() { return {}; } });
+    Object.defineProperty(instance, 'exports', {
+      get() {
+        return { memory: new Uint8Array() };
+      },
+    });
     assert.throws(
       () => { wasi.start(instance); },
       {
@@ -65,7 +69,8 @@ const bufferSource = fixtures.readSync('simple.wasm');
       get() {
         return {
           _start() {},
-          _initialize() {}
+          _initialize() {},
+          memory: new Uint8Array(),
         };
       }
     });