diff --git a/js/deno.ts b/js/deno.ts
index 21766401b6..d121b3183a 100644
--- a/js/deno.ts
+++ b/js/deno.ts
@@ -6,6 +6,7 @@ export {
   exit,
   FileInfo,
   makeTempDirSync,
+  mkdirSync,
   readFileSync,
   statSync,
   lStatSync,
diff --git a/js/os.ts b/js/os.ts
index e669aea9d0..cca1218740 100644
--- a/js/os.ts
+++ b/js/os.ts
@@ -104,6 +104,25 @@ export function makeTempDirSync({
   return path!;
 }
 
+// mkdir creates a new directory with the specified name
+// and permission bits  (before umask).
+export function mkdirSync(path: string, mode = 0o777): void {
+  /* Ideally we could write:
+  const res = send({
+    command: fbs.Command.MKDIR_SYNC,
+    mkdirSyncPath: path,
+    mkdirSyncMode: mode,
+  });
+  */
+  const builder = new flatbuffers.Builder();
+  const path_ = builder.createString(path);
+  fbs.MkdirSync.startMkdirSync(builder);
+  fbs.MkdirSync.addPath(builder, path_);
+  fbs.MkdirSync.addMode(builder, mode);
+  const msg = fbs.MkdirSync.endMkdirSync(builder);
+  send(builder, fbs.Any.MkdirSync, msg);
+}
+
 export function readFileSync(filename: string): Uint8Array {
   /* Ideally we could write
   const res = send({
diff --git a/js/os_test.ts b/js/os_test.ts
index cd5ede221d..8142956ebd 100644
--- a/js/os_test.ts
+++ b/js/os_test.ts
@@ -183,3 +183,24 @@ test(function makeTempDirSyncPerm() {
   assert(err);
   assertEqual(err.name, "deno.PermissionDenied");
 });
+
+testPerm({ write: true }, function mkdirSync() {
+  const path = deno.makeTempDirSync() + "/dir/subdir";
+  deno.mkdirSync(path);
+  const pathInfo = deno.statSync(path);
+  assert(pathInfo.isDirectory());
+});
+
+testPerm({ write: false }, function mkdDirSyncPerm() {
+  let err;
+  try {
+    const path = "/baddir";
+    deno.mkdirSync(path);
+  } catch (err_) {
+    err = err_;
+  }
+  // TODO assert(err instanceof deno.PermissionDenied).
+  assert(err);
+  assertEqual(err.name, "deno.PermissionDenied");
+});
+
diff --git a/src/handlers.rs b/src/handlers.rs
index f583f24000..df77df26a3 100644
--- a/src/handlers.rs
+++ b/src/handlers.rs
@@ -38,6 +38,7 @@ pub extern "C" fn msg_from_js(d: *const DenoC, buf: deno_buf) {
     msg::Any::TimerStart => handle_timer_start,
     msg::Any::TimerClear => handle_timer_clear,
     msg::Any::MakeTempDir => handle_make_temp_dir,
+    msg::Any::MkdirSync => handle_mkdir_sync,
     msg::Any::ReadFileSync => handle_read_file_sync,
     msg::Any::SetEnv => handle_set_env,
     msg::Any::StatSync => handle_stat_sync,
@@ -488,6 +489,30 @@ fn handle_make_temp_dir(
   ))
 }
 
+fn handle_mkdir_sync(
+  d: *const DenoC,
+  base: msg::Base,
+  _builder: &mut FlatBufferBuilder,
+) -> HandlerResult {
+  let msg = base.msg_as_mkdir_sync().unwrap();
+  let path = msg.path().unwrap();
+  // TODO let mode = msg.mode();
+  let deno = from_c(d);
+
+  debug!("handle_mkdir_sync {}", path);
+  if deno.flags.allow_write {
+    // TODO(ry) Use mode.
+    deno_fs::mkdir(Path::new(path))?;
+    Ok(null_buf())
+  } else {
+    let err = std::io::Error::new(
+      std::io::ErrorKind::PermissionDenied,
+      "allow_write is off.",
+    );
+    Err(err.into())
+  }
+}
+
 // Prototype https://github.com/denoland/deno/blob/golang/os.go#L171-L184
 fn handle_read_file_sync(
   _d: *const DenoC,
diff --git a/src/msg.fbs b/src/msg.fbs
index ca2e16af7a..ebb4077f7a 100644
--- a/src/msg.fbs
+++ b/src/msg.fbs
@@ -16,6 +16,7 @@ union Any {
   FetchRes,
   MakeTempDir,
   MakeTempDirRes,
+  MkdirSync,
   ReadFileSync,
   ReadFileSyncRes,
   StatSync,
@@ -166,6 +167,12 @@ table MakeTempDirRes {
   path: string;
 }
 
+table MkdirSync {
+  path: string;
+  mode: uint;
+  // mode specified by https://godoc.org/os#FileMode
+}
+
 table ReadFileSync {
   filename: string;
 }