diff --git a/BUILD.gn b/BUILD.gn
index 167ae82fbd..c48afcf83f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -66,6 +66,7 @@ ts_sources = [
   "js/libdeno.ts",
   "js/main.ts",
   "js/mkdir.ts",
+  "js/make_temp_dir.ts",
   "js/mock_builtin.js",
   "js/os.ts",
   "js/plugins.d.ts",
diff --git a/js/deno.ts b/js/deno.ts
index 187d0f27f7..7ff763e8d7 100644
--- a/js/deno.ts
+++ b/js/deno.ts
@@ -1,8 +1,9 @@
 // Copyright 2018 the Deno authors. All rights reserved. MIT license.
 // Public deno module.
 /// <amd-module name="deno"/>
-export { env, exit, makeTempDirSync } from "./os";
+export { env, exit } from "./os";
 export { mkdirSync, mkdir } from "./mkdir";
+export { makeTempDirSync, makeTempDir } from "./make_temp_dir";
 export { removeSync, remove, removeAllSync, removeAll } from "./remove";
 export { readFileSync, readFile } from "./read_file";
 export { renameSync, rename } from "./rename";
diff --git a/js/make_temp_dir.ts b/js/make_temp_dir.ts
new file mode 100644
index 0000000000..b252be3810
--- /dev/null
+++ b/js/make_temp_dir.ts
@@ -0,0 +1,74 @@
+// Copyright 2018 the Deno authors. All rights reserved. MIT license.
+import * as fbs from "gen/msg_generated";
+import { flatbuffers } from "flatbuffers";
+import * as dispatch from "./dispatch";
+import { assert } from "./util";
+
+export interface MakeTempDirOptions {
+  dir?: string;
+  prefix?: string;
+  suffix?: string;
+}
+
+/**
+ * makeTempDirSync is the synchronous version of `makeTempDir`.
+ *
+ *     import { makeTempDirSync } from "deno";
+ *     const tempDirName0 = makeTempDirSync();
+ *     const tempDirName1 = makeTempDirSync({ prefix: 'my_temp' });
+ */
+export function makeTempDirSync(options: MakeTempDirOptions = {}): string {
+  return res(dispatch.sendSync(...req(options)));
+}
+
+/**
+ * makeTempDir creates a new temporary directory in the directory `dir`, its
+ * name beginning with `prefix` and ending with `suffix`.
+ * It returns the full path to the newly created directory.
+ * If `dir` is unspecified, tempDir uses the default directory for temporary
+ * files. Multiple programs calling tempDir simultaneously will not choose the
+ * same directory. It is the caller's responsibility to remove the directory
+ * when no longer needed.
+ *
+ *     import { makeTempDir } from "deno";
+ *     const tempDirName0 = await makeTempDir();
+ *     const tempDirName1 = await makeTempDir({ prefix: 'my_temp' });
+ */
+export async function makeTempDir(
+  options: MakeTempDirOptions = {}
+): Promise<string> {
+  return res(await dispatch.sendAsync(...req(options)));
+}
+
+function req({
+  dir,
+  prefix,
+  suffix
+}: MakeTempDirOptions): [flatbuffers.Builder, fbs.Any, flatbuffers.Offset] {
+  const builder = new flatbuffers.Builder();
+  const fbDir = dir == null ? -1 : builder.createString(dir);
+  const fbPrefix = prefix == null ? -1 : builder.createString(prefix);
+  const fbSuffix = suffix == null ? -1 : builder.createString(suffix);
+  fbs.MakeTempDir.startMakeTempDir(builder);
+  if (dir != null) {
+    fbs.MakeTempDir.addDir(builder, fbDir);
+  }
+  if (prefix != null) {
+    fbs.MakeTempDir.addPrefix(builder, fbPrefix);
+  }
+  if (suffix != null) {
+    fbs.MakeTempDir.addSuffix(builder, fbSuffix);
+  }
+  const msg = fbs.MakeTempDir.endMakeTempDir(builder);
+  return [builder, fbs.Any.MakeTempDir, msg];
+}
+
+function res(baseRes: null | fbs.Base): string {
+  assert(baseRes != null);
+  assert(fbs.Any.MakeTempDirRes === baseRes!.msgType());
+  const res = new fbs.MakeTempDirRes();
+  assert(baseRes!.msg(res) != null);
+  const path = res.path();
+  assert(path != null);
+  return path!;
+}
diff --git a/js/make_temp_dir_test.ts b/js/make_temp_dir_test.ts
new file mode 100644
index 0000000000..082c1e65d2
--- /dev/null
+++ b/js/make_temp_dir_test.ts
@@ -0,0 +1,67 @@
+// Copyright 2018 the Deno authors. All rights reserved. MIT license.
+import { test, testPerm, assert, assertEqual } from "./test_util.ts";
+import * as deno from "deno";
+
+testPerm({ write: true }, function makeTempDirSyncSuccess() {
+  const dir1 = deno.makeTempDirSync({ prefix: "hello", suffix: "world" });
+  const dir2 = deno.makeTempDirSync({ prefix: "hello", suffix: "world" });
+  // Check that both dirs are different.
+  assert(dir1 != dir2);
+  for (const dir of [dir1, dir2]) {
+    // Check that the prefix and suffix are applied.
+    const lastPart = dir.replace(/^.*[\\\/]/, "");
+    assert(lastPart.startsWith("hello"));
+    assert(lastPart.endsWith("world"));
+  }
+  // Check that the `dir` option works.
+  const dir3 = deno.makeTempDirSync({ dir: dir1 });
+  assert(dir3.startsWith(dir1));
+  assert(/^[\\\/]/.test(dir3.slice(dir1.length)));
+  // Check that creating a temp dir inside a nonexisting directory fails.
+  let err;
+  try {
+    deno.makeTempDirSync({ dir: "/baddir" });
+  } catch (err_) {
+    err = err_;
+  }
+  assertEqual(err.kind, deno.ErrorKind.NotFound);
+  assertEqual(err.name, "NotFound");
+});
+
+test(function makeTempDirSyncPerm() {
+  // makeTempDirSync should require write permissions (for now).
+  let err;
+  try {
+    deno.makeTempDirSync({ dir: "/baddir" });
+  } catch (err_) {
+    err = err_;
+  }
+  assertEqual(err.kind, deno.ErrorKind.PermissionDenied);
+  assertEqual(err.name, "PermissionDenied");
+});
+
+testPerm({ write: true }, async function makeTempDirSuccess() {
+  const dir1 = await deno.makeTempDir({ prefix: "hello", suffix: "world" });
+  const dir2 = await deno.makeTempDir({ prefix: "hello", suffix: "world" });
+  // Check that both dirs are different.
+  assert(dir1 != dir2);
+  for (const dir of [dir1, dir2]) {
+    // Check that the prefix and suffix are applied.
+    const lastPart = dir.replace(/^.*[\\\/]/, "");
+    assert(lastPart.startsWith("hello"));
+    assert(lastPart.endsWith("world"));
+  }
+  // Check that the `dir` option works.
+  const dir3 = await deno.makeTempDir({ dir: dir1 });
+  assert(dir3.startsWith(dir1));
+  assert(/^[\\\/]/.test(dir3.slice(dir1.length)));
+  // Check that creating a temp dir inside a nonexisting directory fails.
+  let err;
+  try {
+    await deno.makeTempDir({ dir: "/baddir" });
+  } catch (err_) {
+    err = err_;
+  }
+  assertEqual(err.kind, deno.ErrorKind.NotFound);
+  assertEqual(err.name, "NotFound");
+});
diff --git a/js/os.ts b/js/os.ts
index d73865c065..020d834c6b 100644
--- a/js/os.ts
+++ b/js/os.ts
@@ -63,50 +63,6 @@ export function codeCache(
   assert(baseRes == null); // Expect null or error.
 }
 
-/**
- * makeTempDirSync creates a new temporary directory in the directory `dir`, its
- * name beginning with `prefix` and ending with `suffix`.
- * It returns the full path to the newly created directory.
- * If `dir` is unspecified, tempDir uses the default directory for temporary
- * files. Multiple programs calling tempDir simultaneously will not choose the
- * same directory. It is the caller's responsibility to remove the directory
- * when no longer needed.
- */
-export interface MakeTempDirOptions {
-  dir?: string;
-  prefix?: string;
-  suffix?: string;
-}
-export function makeTempDirSync({
-  dir,
-  prefix,
-  suffix
-}: MakeTempDirOptions = {}): string {
-  const builder = new flatbuffers.Builder();
-  const fbDir = dir == null ? -1 : builder.createString(dir);
-  const fbPrefix = prefix == null ? -1 : builder.createString(prefix);
-  const fbSuffix = suffix == null ? -1 : builder.createString(suffix);
-  fbs.MakeTempDir.startMakeTempDir(builder);
-  if (dir != null) {
-    fbs.MakeTempDir.addDir(builder, fbDir);
-  }
-  if (prefix != null) {
-    fbs.MakeTempDir.addPrefix(builder, fbPrefix);
-  }
-  if (suffix != null) {
-    fbs.MakeTempDir.addSuffix(builder, fbSuffix);
-  }
-  const msg = fbs.MakeTempDir.endMakeTempDir(builder);
-  const baseRes = sendSync(builder, fbs.Any.MakeTempDir, msg);
-  assert(baseRes != null);
-  assert(fbs.Any.MakeTempDirRes === baseRes!.msgType());
-  const res = new fbs.MakeTempDirRes();
-  assert(baseRes!.msg(res) != null);
-  const path = res.path();
-  assert(path != null);
-  return path!;
-}
-
 function createEnv(_msg: fbs.EnvironRes): { [index: string]: string } {
   const env: { [index: string]: string } = {};
 
diff --git a/js/os_test.ts b/js/os_test.ts
index 61227a3b10..922d86850e 100644
--- a/js/os_test.ts
+++ b/js/os_test.ts
@@ -22,41 +22,3 @@ test(async function envFailure() {
 
   assert(caughtError);
 });
-
-testPerm({ write: true }, function makeTempDirSync() {
-  const dir1 = deno.makeTempDirSync({ prefix: "hello", suffix: "world" });
-  const dir2 = deno.makeTempDirSync({ prefix: "hello", suffix: "world" });
-  // Check that both dirs are different.
-  assert(dir1 != dir2);
-  for (const dir of [dir1, dir2]) {
-    // Check that the prefix and suffix are applied.
-    const lastPart = dir.replace(/^.*[\\\/]/, "");
-    assert(lastPart.startsWith("hello"));
-    assert(lastPart.endsWith("world"));
-  }
-  // Check that the `dir` option works.
-  const dir3 = deno.makeTempDirSync({ dir: dir1 });
-  assert(dir3.startsWith(dir1));
-  assert(/^[\\\/]/.test(dir3.slice(dir1.length)));
-  // Check that creating a temp dir inside a nonexisting directory fails.
-  let err;
-  try {
-    deno.makeTempDirSync({ dir: "/baddir" });
-  } catch (err_) {
-    err = err_;
-  }
-  assertEqual(err.kind, deno.ErrorKind.NotFound);
-  assertEqual(err.name, "NotFound");
-});
-
-test(function makeTempDirSyncPerm() {
-  // makeTempDirSync should require write permissions (for now).
-  let err;
-  try {
-    deno.makeTempDirSync({ dir: "/baddir" });
-  } catch (err_) {
-    err = err_;
-  }
-  assertEqual(err.kind, deno.ErrorKind.PermissionDenied);
-  assertEqual(err.name, "PermissionDenied");
-});
diff --git a/js/unit_tests.ts b/js/unit_tests.ts
index b406b63d08..be947cd675 100644
--- a/js/unit_tests.ts
+++ b/js/unit_tests.ts
@@ -8,5 +8,6 @@ import "./os_test.ts";
 import "./read_file_test.ts";
 import "./write_file_test.ts";
 import "./mkdir_test.ts";
+import "./make_temp_dir_test.ts";
 import "./stat_test.ts";
 import "./rename_test.ts";