diff --git a/deno.d.ts b/deno.d.ts
index 5113cf412b..59b1f2dfd1 100644
--- a/deno.d.ts
+++ b/deno.d.ts
@@ -4,4 +4,9 @@ declare module "deno" {
   function pub(channel: string, payload: Uint8Array): null | ArrayBuffer;
 
   function readFileSync(filename: string): Uint8Array;
+  function writeFileSync(
+    filename: string,
+    data: Uint8Array,
+    perm: number
+  ): void;
 }
diff --git a/msg.proto b/msg.proto
index ecd6366184..1a2abcbbff 100644
--- a/msg.proto
+++ b/msg.proto
@@ -21,6 +21,7 @@ message Msg {
     FETCH_RES = 10;
     READ_FILE_SYNC = 11;
     READ_FILE_SYNC_RES = 12;
+    WRITE_FILE_SYNC = 13;
   }
   Command command = 1;
 
@@ -89,4 +90,10 @@ message Msg {
 
   // READ_FILE_SYNC_RES
   bytes read_file_sync_data = 120;
+
+  // WRITE_FILE_SYNC
+  string write_file_sync_filename = 130;
+  bytes write_file_sync_data = 131;
+  uint32 write_file_sync_perm = 132;
+  // write_file_sync_perm specified by https://godoc.org/os#FileMode
 }
diff --git a/os.go b/os.go
index 2fbbd95a53..52c70eddcd 100644
--- a/os.go
+++ b/os.go
@@ -34,6 +34,9 @@ func InitOS() {
 			os.Exit(int(msg.ExitCode))
 		case Msg_READ_FILE_SYNC:
 			return ReadFileSync(msg.ReadFileSyncFilename)
+		case Msg_WRITE_FILE_SYNC:
+			return WriteFileSync(msg.WriteFileSyncFilename, msg.WriteFileSyncData,
+				msg.WriteFileSyncPerm)
 		default:
 			panic("[os] Unexpected message " + string(buf))
 		}
@@ -176,3 +179,14 @@ func ReadFileSync(filename string) []byte {
 	check(err)
 	return out
 }
+
+func WriteFileSync(filename string, data []byte, perm uint32) []byte {
+	err := afero.WriteFile(fs, filename, data, os.FileMode(perm))
+	res := &Msg{}
+	if err != nil {
+		res.Error = err.Error()
+	}
+	out, err := proto.Marshal(res)
+	check(err)
+	return out
+}
diff --git a/os.ts b/os.ts
index 8628edd2a1..18ccb5bb5a 100644
--- a/os.ts
+++ b/os.ts
@@ -48,3 +48,16 @@ export function readFileSync(filename: string): Uint8Array {
   });
   return res.readFileSyncData;
 }
+
+export function writeFileSync(
+  filename: string,
+  data: Uint8Array,
+  perm: number
+): void {
+  sendMsg("os", {
+    command: pb.Msg.Command.WRITE_FILE_SYNC,
+    writeFileSyncFilename: filename,
+    writeFileSyncData: data,
+    writeFileSyncPerm: perm
+  });
+}
diff --git a/runtime.ts b/runtime.ts
index 7d91484ab5..e46c73c61e 100644
--- a/runtime.ts
+++ b/runtime.ts
@@ -20,7 +20,8 @@ const EOL = "\n";
 const deno = {
   pub,
   sub,
-  readFileSync: os.readFileSync
+  readFileSync: os.readFileSync,
+  writeFileSync: os.writeFileSync
 };
 
 // tslint:disable-next-line:no-any
diff --git a/tests.ts b/tests.ts
index 702f6406d4..b2c7ee5135 100644
--- a/tests.ts
+++ b/tests.ts
@@ -2,7 +2,7 @@
 // But it can also be run manually:
 //  ./deno tests.ts
 import { test, assert, assertEqual } from "./deno_testing/testing.ts";
-import { readFileSync } from "deno";
+import { readFileSync, writeFileSync } from "deno";
 
 test(async function tests_test() {
   assert(true);
@@ -21,8 +21,20 @@ test(async function tests_readFileSync() {
   assertEqual(pkg.name, "deno");
 });
 
+test(async function tests_writeFileSync() {
+  const enc = new TextEncoder();
+  const data = enc.encode("Hello");
+  // TODO need ability to get tmp dir.
+  const fn = "/tmp/test.txt";
+  writeFileSync("/tmp/test.txt", data, 0o666);
+  const dataRead = readFileSync("/tmp/test.txt");
+  const dec = new TextDecoder("utf-8");
+  const actual = dec.decode(dataRead);
+  assertEqual("Hello", actual);
+});
+
 test(async function tests_fetch() {
-  const response = await fetch('http://localhost:4545/package.json');
+  const response = await fetch("http://localhost:4545/package.json");
   const json = await response.json();
   assertEqual(json.name, "deno");
 });