diff --git a/cli/main.rs b/cli/main.rs
index d8d834e8f74812..ff757ed3664ad4 100644
--- a/cli/main.rs
+++ b/cli/main.rs
@@ -196,8 +196,10 @@ fn create_worker_and_state(
       s.status(status, msg).expect("shell problem");
     }
   });
+  // TODO(kevinkassimo): maybe make include_deno_namespace also configurable?
   let state =
-    ThreadSafeState::new(flags, argv, ops::op_selector_std, progress).unwrap();
+    ThreadSafeState::new(flags, argv, ops::op_selector_std, progress, true)
+      .unwrap();
   let worker = Worker::new(
     "main".to_string(),
     startup_data::deno_isolate_init(),
diff --git a/cli/msg.fbs b/cli/msg.fbs
index 014bfb7e939bff..8f6497afcbcb2e 100644
--- a/cli/msg.fbs
+++ b/cli/msg.fbs
@@ -204,6 +204,7 @@ table FormatErrorRes {
 // Create worker as host
 table CreateWorker {
   specifier: string;
+  include_deno_namespace: bool;
 }
 
 table CreateWorkerRes {
diff --git a/cli/ops.rs b/cli/ops.rs
index 0e90e19c42c473..06c4eae99853f2 100644
--- a/cli/ops.rs
+++ b/cli/ops.rs
@@ -2069,6 +2069,10 @@ fn op_create_worker(
   let cmd_id = base.cmd_id();
   let inner = base.inner_as_create_worker().unwrap();
   let specifier = inner.specifier().unwrap();
+  // Only include deno namespace if requested AND current worker
+  // has included namespace (to avoid escalation).
+  let include_deno_namespace =
+    inner.include_deno_namespace() && state.include_deno_namespace;
 
   let parent_state = state.clone();
 
@@ -2077,13 +2081,15 @@ fn op_create_worker(
     parent_state.argv.clone(),
     op_selector_std,
     parent_state.progress.clone(),
+    include_deno_namespace,
   )?;
   let rid = child_state.resource.rid;
   let name = format!("USER-WORKER-{}", specifier);
+  let deno_main_call = format!("denoMain({})", include_deno_namespace);
 
   let mut worker =
     Worker::new(name, startup_data::deno_isolate_init(), child_state);
-  worker.execute("denoMain()").unwrap();
+  worker.execute(&deno_main_call).unwrap();
   worker.execute("workerMain()").unwrap();
 
   let module_specifier = ModuleSpecifier::resolve_url_or_path(specifier)?;
diff --git a/cli/state.rs b/cli/state.rs
index 139584394a7c31..f4e3d9c84164a8 100644
--- a/cli/state.rs
+++ b/cli/state.rs
@@ -84,6 +84,8 @@ pub struct State {
   pub js_compiler: JsCompiler,
   pub json_compiler: JsonCompiler,
   pub ts_compiler: TsCompiler,
+
+  pub include_deno_namespace: bool,
 }
 
 impl Clone for ThreadSafeState {
@@ -151,6 +153,7 @@ impl ThreadSafeState {
     argv_rest: Vec<String>,
     dispatch_selector: ops::OpSelector,
     progress: Progress,
+    include_deno_namespace: bool,
   ) -> Result<Self, ErrBox> {
     let custom_root = env::var("DENO_DIR").map(String::into).ok();
 
@@ -223,6 +226,7 @@ impl ThreadSafeState {
       ts_compiler,
       js_compiler: JsCompiler {},
       json_compiler: JsonCompiler {},
+      include_deno_namespace,
     };
 
     Ok(ThreadSafeState(Arc::new(state)))
@@ -302,6 +306,7 @@ impl ThreadSafeState {
       argv,
       ops::op_selector_std,
       Progress::new(),
+      true,
     )
     .unwrap()
   }
diff --git a/cli/worker.rs b/cli/worker.rs
index befc69f854ee60..f1806283692822 100644
--- a/cli/worker.rs
+++ b/cli/worker.rs
@@ -126,6 +126,7 @@ mod tests {
       argv,
       op_selector_std,
       Progress::new(),
+      true,
     )
     .unwrap();
     let state_ = state.clone();
@@ -155,6 +156,7 @@ mod tests {
       argv,
       op_selector_std,
       Progress::new(),
+      true,
     )
     .unwrap();
     let state_ = state.clone();
@@ -182,7 +184,7 @@ mod tests {
     let mut flags = flags::DenoFlags::default();
     flags.reload = true;
     let state =
-      ThreadSafeState::new(flags, argv, op_selector_std, Progress::new())
+      ThreadSafeState::new(flags, argv, op_selector_std, Progress::new(), true)
         .unwrap();
     let state_ = state.clone();
     tokio_util::run(lazy(move || {
diff --git a/core/shared_queue.js b/core/shared_queue.js
index 75f370ce44fe57..d7ab382d6e9317 100644
--- a/core/shared_queue.js
+++ b/core/shared_queue.js
@@ -29,8 +29,12 @@ SharedQueue Binary Layout
   const INDEX_RECORDS = 3 + MAX_RECORDS;
   const HEAD_INIT = 4 * INDEX_RECORDS;
 
+  // Available on start due to bindings.
   const Deno = window[GLOBAL_NAMESPACE];
   const core = Deno[CORE_NAMESPACE];
+  // Warning: DO NOT use window.Deno after this point.
+  // It is possible that the Deno namespace has been deleted.
+  // Use the above local Deno and core variable instead.
 
   let sharedBytes;
   let shared32;
@@ -165,7 +169,7 @@ SharedQueue Binary Layout
     const success = push(control);
     // If successful, don't use first argument of core.send.
     const arg0 = success ? null : control;
-    return window.Deno.core.send(arg0, zeroCopy);
+    return Deno.core.send(arg0, zeroCopy);
   }
 
   const denoCore = {
diff --git a/js/compiler.ts b/js/compiler.ts
index 4203f753ba9bb6..6bda5a1eca970e 100644
--- a/js/compiler.ts
+++ b/js/compiler.ts
@@ -25,7 +25,7 @@ const console = new Console(core.print);
 window.console = console;
 window.workerMain = workerMain;
 export default function denoMain(): void {
-  os.start("TS");
+  os.start(true, "TS");
 }
 
 const ASSETS = "$asset$";
diff --git a/js/core.ts b/js/core.ts
index 03473ff3dc0c5d..436ba7ccbe0c25 100644
--- a/js/core.ts
+++ b/js/core.ts
@@ -1,4 +1,6 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 import { window } from "./window";
 
+// This allows us to access core in API even if we
+// dispose window.Deno
 export const core = window.Deno.core as DenoCore;
diff --git a/js/globals.ts b/js/globals.ts
index 27cef39098dd3e..92e4662aa7fef4 100644
--- a/js/globals.ts
+++ b/js/globals.ts
@@ -32,7 +32,6 @@ import * as request from "./request";
 // These imports are not exposed and therefore are fine to just import the
 // symbols required.
 import { core } from "./core";
-import { immutableDefine } from "./util";
 
 // During the build process, augmentations to the variable `window` in this
 // file are tracked and created as part of default library that is built into
@@ -71,7 +70,7 @@ window.window = window;
 // This is the Deno namespace, it is handled differently from other window
 // properties when building the runtime type library, as the whole module
 // is flattened into a single namespace.
-immutableDefine(window, "Deno", deno);
+window.Deno = deno;
 Object.freeze(window.Deno);
 
 // Globally available functions and object instances.
diff --git a/js/main.ts b/js/main.ts
index 5687a69268e517..769f522a63c312 100644
--- a/js/main.ts
+++ b/js/main.ts
@@ -18,8 +18,11 @@ import { setLocation } from "./location";
 // builtin modules
 import * as deno from "./deno";
 
-export default function denoMain(name?: string): void {
-  const startResMsg = os.start(name);
+export default function denoMain(
+  preserveDenoNamespace: boolean = true,
+  name?: string
+): void {
+  const startResMsg = os.start(preserveDenoNamespace, name);
 
   setVersions(startResMsg.denoVersion()!, startResMsg.v8Version()!);
 
diff --git a/js/os.ts b/js/os.ts
index 558f47efd7cade..e7d588a52fd555 100644
--- a/js/os.ts
+++ b/js/os.ts
@@ -112,7 +112,10 @@ function sendStart(): msg.StartRes {
 // This function bootstraps an environment within Deno, it is shared both by
 // the runtime and the compiler environments.
 // @internal
-export function start(source?: string): msg.StartRes {
+export function start(
+  preserveDenoNamespace = true,
+  source?: string
+): msg.StartRes {
   core.setAsyncHandler(handleAsyncMsgFromRust);
 
   // First we send an empty `Start` message to let the privileged side know we
@@ -124,9 +127,18 @@ export function start(source?: string): msg.StartRes {
 
   setGlobals(startResMsg.pid(), startResMsg.noColor(), startResMsg.execPath()!);
 
-  // Deno.core could ONLY be safely frozen here (not in globals.ts)
-  // since shared_queue.js will modify core properties.
-  Object.freeze(window.Deno.core);
+  if (preserveDenoNamespace) {
+    util.immutableDefine(window, "Deno", window.Deno);
+    // Deno.core could ONLY be safely frozen here (not in globals.ts)
+    // since shared_queue.js will modify core properties.
+    Object.freeze(window.Deno.core);
+    // core.sharedQueue is an object so we should also freeze it.
+    Object.freeze(window.Deno.core.sharedQueue);
+  } else {
+    // Remove window.Deno
+    delete window.Deno;
+    assert(window.Deno === undefined);
+  }
 
   return startResMsg;
 }
diff --git a/js/workers.ts b/js/workers.ts
index 1531b960b0aa8c..f008ccecf5ef01 100644
--- a/js/workers.ts
+++ b/js/workers.ts
@@ -20,10 +20,17 @@ export function decodeMessage(dataIntArray: Uint8Array): any {
   return JSON.parse(dataJson);
 }
 
-function createWorker(specifier: string): number {
+function createWorker(
+  specifier: string,
+  includeDenoNamespace: boolean
+): number {
   const builder = flatbuffers.createBuilder();
   const specifier_ = builder.createString(specifier);
-  const inner = msg.CreateWorker.createCreateWorker(builder, specifier_);
+  const inner = msg.CreateWorker.createCreateWorker(
+    builder,
+    specifier_,
+    includeDenoNamespace
+  );
   const baseRes = sendSync(builder, msg.Any.CreateWorker, inner);
   assert(baseRes != null);
   assert(
@@ -149,6 +156,18 @@ export interface Worker {
   closed: Promise<void>;
 }
 
+// TODO(kevinkassimo): Maybe implement reasonable web worker options?
+// eslint-disable-next-line @typescript-eslint/no-empty-interface
+export interface WorkerOptions {}
+
+/** Extended Deno Worker initialization options.
+ * `noDenoNamespace` hides global `window.Deno` namespace for
+ * spawned worker and nested workers spawned by it (default: false).
+ */
+export interface DenoWorkerOptions extends WorkerOptions {
+  noDenoNamespace?: boolean;
+}
+
 export class WorkerImpl implements Worker {
   private readonly rid: number;
   private isClosing: boolean = false;
@@ -157,8 +176,12 @@ export class WorkerImpl implements Worker {
   public onmessage?: (data: any) => void;
   public onmessageerror?: () => void;
 
-  constructor(specifier: string) {
-    this.rid = createWorker(specifier);
+  constructor(specifier: string, options?: DenoWorkerOptions) {
+    let includeDenoNamespace = true;
+    if (options && options.noDenoNamespace) {
+      includeDenoNamespace = false;
+    }
+    this.rid = createWorker(specifier, includeDenoNamespace);
     this.run();
     this.isClosedPromise = hostGetWorkerClosed(this.rid);
     this.isClosedPromise.then(
diff --git a/tests/039_worker_deno_ns.test b/tests/039_worker_deno_ns.test
new file mode 100644
index 00000000000000..da282cc7b1a95e
--- /dev/null
+++ b/tests/039_worker_deno_ns.test
@@ -0,0 +1,2 @@
+args: run --reload tests/039_worker_deno_ns.ts
+output: tests/039_worker_deno_ns.ts.out
diff --git a/tests/039_worker_deno_ns.ts b/tests/039_worker_deno_ns.ts
new file mode 100644
index 00000000000000..1c6830c2268826
--- /dev/null
+++ b/tests/039_worker_deno_ns.ts
@@ -0,0 +1,25 @@
+const w1 = new Worker("./tests/039_worker_deno_ns/has_ns.ts");
+const w2 = new Worker("./tests/039_worker_deno_ns/no_ns.ts", {
+  noDenoNamespace: true
+});
+let w1MsgCount = 0;
+let w2MsgCount = 0;
+w1.onmessage = (msg): void => {
+  console.log(msg.data);
+  w1MsgCount++;
+  if (w1MsgCount === 1) {
+    w1.postMessage("CONTINUE");
+  } else {
+    w2.postMessage("START");
+  }
+};
+w2.onmessage = (msg): void => {
+  console.log(msg.data);
+  w2MsgCount++;
+  if (w2MsgCount === 1) {
+    w2.postMessage("CONTINUE");
+  } else {
+    Deno.exit(0);
+  }
+};
+w1.postMessage("START");
diff --git a/tests/039_worker_deno_ns.ts.out b/tests/039_worker_deno_ns.ts.out
new file mode 100644
index 00000000000000..9b2f90099cceab
--- /dev/null
+++ b/tests/039_worker_deno_ns.ts.out
@@ -0,0 +1,4 @@
+has_ns.ts: is window.Deno available: true
+[SPAWNED BY has_ns.ts] maybe_ns.ts: is window.Deno available: true
+no_ns.ts: is window.Deno available: false
+[SPAWNED BY no_ns.ts] maybe_ns.ts: is window.Deno available: false
diff --git a/tests/039_worker_deno_ns/has_ns.ts b/tests/039_worker_deno_ns/has_ns.ts
new file mode 100644
index 00000000000000..15e729f63a73cd
--- /dev/null
+++ b/tests/039_worker_deno_ns/has_ns.ts
@@ -0,0 +1,10 @@
+onmessage = (msg): void => {
+  if (msg.data === "START") {
+    postMessage("has_ns.ts: is window.Deno available: " + !!window.Deno);
+  } else {
+    const worker = new Worker("./tests/039_worker_deno_ns/maybe_ns.ts");
+    worker.onmessage = (msg): void => {
+      postMessage("[SPAWNED BY has_ns.ts] " + msg.data);
+    };
+  }
+};
diff --git a/tests/039_worker_deno_ns/maybe_ns.ts b/tests/039_worker_deno_ns/maybe_ns.ts
new file mode 100644
index 00000000000000..0bcbd1f973c816
--- /dev/null
+++ b/tests/039_worker_deno_ns/maybe_ns.ts
@@ -0,0 +1 @@
+postMessage("maybe_ns.ts: is window.Deno available: " + !!window.Deno);
diff --git a/tests/039_worker_deno_ns/no_ns.ts b/tests/039_worker_deno_ns/no_ns.ts
new file mode 100644
index 00000000000000..4cee98bea31649
--- /dev/null
+++ b/tests/039_worker_deno_ns/no_ns.ts
@@ -0,0 +1,10 @@
+onmessage = (msg): void => {
+  if (msg.data === "START") {
+    postMessage("no_ns.ts: is window.Deno available: " + !!window.Deno);
+  } else {
+    const worker = new Worker("./tests/039_worker_deno_ns/maybe_ns.ts");
+    worker.onmessage = (msg): void => {
+      postMessage("[SPAWNED BY no_ns.ts] " + msg.data);
+    };
+  }
+};