diff --git a/cli/js/deno.ts b/cli/js/deno.ts
index 23fcfda5d8..f0115f2612 100644
--- a/cli/js/deno.ts
+++ b/cli/js/deno.ts
@@ -113,6 +113,7 @@ export { symlinkSync, symlink } from "./ops/fs/symlink.ts";
 export { connectTLS, listenTLS } from "./tls.ts";
 export { truncateSync, truncate } from "./ops/fs/truncate.ts";
 export { isatty, setRaw } from "./ops/tty.ts";
+export { umask } from "./ops/fs/umask.ts";
 export { utimeSync, utime } from "./ops/fs/utime.ts";
 export { version } from "./version.ts";
 export { writeFileSync, writeFile, WriteFileOptions } from "./write_file.ts";
diff --git a/cli/js/lib.deno.ns.d.ts b/cli/js/lib.deno.ns.d.ts
index 8eb2e1f4d8..6950d90500 100644
--- a/cli/js/lib.deno.ns.d.ts
+++ b/cli/js/lib.deno.ns.d.ts
@@ -287,6 +287,14 @@ declare namespace Deno {
    */
   export function chdir(directory: string): void;
 
+  /**
+   * **UNSTABLE**: New API. Maybe needs permissions.
+   *
+   * If `mask` is provided, sets the process umask. Always returns what the umask
+   * was before the call.
+   */
+  export function umask(mask?: number): number;
+
   /** **UNSTABLE**: might move to `Deno.symbols`. */
   export const EOF: unique symbol;
   export type EOF = typeof EOF;
diff --git a/cli/js/ops/fs/umask.ts b/cli/js/ops/fs/umask.ts
new file mode 100644
index 0000000000..ee56fecb56
--- /dev/null
+++ b/cli/js/ops/fs/umask.ts
@@ -0,0 +1,12 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+import { sendSync } from "../dispatch_json.ts";
+
+/**
+ * **UNSTABLE**: maybe needs `allow-env` permissions.
+ *
+ * If `mask` is provided, sets the process umask. Always returns what the umask
+ * was before the call.
+ */
+export function umask(mask?: number): number {
+  return sendSync("op_umask", { mask });
+}
diff --git a/cli/js/tests/umask_test.ts b/cli/js/tests/umask_test.ts
new file mode 100644
index 0000000000..543372a464
--- /dev/null
+++ b/cli/js/tests/umask_test.ts
@@ -0,0 +1,16 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+import { unitTest, assertEquals } from "./test_util.ts";
+
+unitTest(
+  {
+    skip: Deno.build.os === "win"
+  },
+  function umaskSuccess(): void {
+    const prevMask = Deno.umask(0o020);
+    const newMask = Deno.umask(prevMask);
+    const finalMask = Deno.umask();
+    assertEquals(newMask, 0o020);
+    assertEquals(finalMask, prevMask);
+    assertEquals(prevMask, 0o022);
+  }
+);
diff --git a/cli/js/tests/unit_tests.ts b/cli/js/tests/unit_tests.ts
index cc51e6ade5..9ae724d535 100644
--- a/cli/js/tests/unit_tests.ts
+++ b/cli/js/tests/unit_tests.ts
@@ -57,6 +57,7 @@ import "./timers_test.ts";
 import "./tls_test.ts";
 import "./truncate_test.ts";
 import "./tty_test.ts";
+import "./umask_test.ts";
 import "./url_test.ts";
 import "./url_search_params_test.ts";
 import "./utime_test.ts";
diff --git a/cli/op_error.rs b/cli/op_error.rs
index a2cf03a661..bebfd2e723 100644
--- a/cli/op_error.rs
+++ b/cli/op_error.rs
@@ -71,6 +71,10 @@ impl OpError {
     Self::new(ErrorKind::NotFound, msg)
   }
 
+  pub fn not_implemented() -> Self {
+    Self::other("not implemented".to_string())
+  }
+
   pub fn other(msg: String) -> Self {
     Self::new(ErrorKind::Other, msg)
   }
diff --git a/cli/ops/fs.rs b/cli/ops/fs.rs
index ad1283c122..df5ad7bef1 100644
--- a/cli/ops/fs.rs
+++ b/cli/ops/fs.rs
@@ -23,6 +23,7 @@ use std::os::unix::fs::{MetadataExt, PermissionsExt};
 pub fn init(i: &mut Isolate, s: &State) {
   i.register_op("op_open", s.stateful_json_op(op_open));
   i.register_op("op_seek", s.stateful_json_op(op_seek));
+  i.register_op("op_umask", s.stateful_json_op(op_umask));
   i.register_op("op_chdir", s.stateful_json_op(op_chdir));
   i.register_op("op_mkdir", s.stateful_json_op(op_mkdir));
   i.register_op("op_chmod", s.stateful_json_op(op_chmod));
@@ -221,6 +222,43 @@ fn op_seek(
   }
 }
 
+#[derive(Deserialize)]
+struct UmaskArgs {
+  mask: Option<u32>,
+}
+
+fn op_umask(
+  _state: &State,
+  args: Value,
+  _zero_copy: Option<ZeroCopyBuf>,
+) -> Result<JsonOp, OpError> {
+  let args: UmaskArgs = serde_json::from_value(args)?;
+  // TODO implement umask for Windows
+  // see https://github.com/nodejs/node/blob/master/src/node_process_methods.cc
+  // and https://docs.microsoft.com/fr-fr/cpp/c-runtime-library/reference/umask?view=vs-2019
+  #[cfg(not(unix))]
+  {
+    let _ = args.mask; // avoid unused warning.
+    return Err(OpError::not_implemented());
+  }
+  #[cfg(unix)]
+  {
+    use nix::sys::stat::mode_t;
+    use nix::sys::stat::umask;
+    use nix::sys::stat::Mode;
+    let r = if let Some(mask) = args.mask {
+      // If mask provided, return previous.
+      umask(Mode::from_bits_truncate(mask as mode_t))
+    } else {
+      // If no mask provided, we query the current. Requires two syscalls.
+      let prev = umask(Mode::from_bits_truncate(0o777));
+      let _ = umask(prev);
+      prev
+    };
+    Ok(JsonOp::Sync(json!(r.bits() as u32)))
+  }
+}
+
 #[derive(Deserialize)]
 struct ChdirArgs {
   directory: String,