diff --git a/cli/dts/lib.deno.unstable.d.ts b/cli/dts/lib.deno.unstable.d.ts
index 2dcd94f75f..4269d4b74d 100644
--- a/cli/dts/lib.deno.unstable.d.ts
+++ b/cli/dts/lib.deno.unstable.d.ts
@@ -1312,6 +1312,17 @@ declare namespace Deno {
     atime: number | Date,
     mtime: number | Date,
   ): Promise<void>;
+
+  /** *UNSTABLE**: new API, yet to be vetted.
+   *
+   * SleepSync puts the main thread to sleep synchronously for a given amount of
+   * time in milliseconds.
+   *
+   * ```ts
+   * Deno.sleepSync(10);
+   * ```
+   */
+  export function sleepSync(millis: number): Promise<void>;
 }
 
 declare function fetch(
diff --git a/cli/ops/timers.rs b/cli/ops/timers.rs
index 74edc7267e..841cdf289d 100644
--- a/cli/ops/timers.rs
+++ b/cli/ops/timers.rs
@@ -25,6 +25,7 @@ use std::cell::RefCell;
 use std::future::Future;
 use std::pin::Pin;
 use std::rc::Rc;
+use std::thread::sleep;
 use std::time::Duration;
 use std::time::Instant;
 
@@ -77,6 +78,7 @@ pub fn init(rt: &mut deno_core::JsRuntime) {
   super::reg_json_sync(rt, "op_global_timer_start", op_global_timer_start);
   super::reg_json_async(rt, "op_global_timer", op_global_timer);
   super::reg_json_sync(rt, "op_now", op_now);
+  super::reg_json_sync(rt, "op_sleep_sync", op_sleep_sync);
 }
 
 fn op_global_timer_stop(
@@ -157,3 +159,19 @@ fn op_now(
     "subsecNanos": subsec_nanos,
   }))
 }
+
+#[derive(Deserialize)]
+struct SleepArgs {
+  millis: u64,
+}
+
+fn op_sleep_sync(
+  state: &mut OpState,
+  args: Value,
+  _zero_copy: &mut [ZeroCopyBuf],
+) -> Result<Value, AnyError> {
+  super::check_unstable(state, "Deno.sleepSync");
+  let args: SleepArgs = serde_json::from_value(args)?;
+  sleep(Duration::from_millis(args.millis));
+  Ok(json!({}))
+}
diff --git a/cli/rt/11_timers.js b/cli/rt/11_timers.js
index 8f6a7e0497..c762c59d83 100644
--- a/cli/rt/11_timers.js
+++ b/cli/rt/11_timers.js
@@ -20,6 +20,10 @@
     return core.jsonOpSync("op_now");
   }
 
+  function sleepSync(millis = 0) {
+    return core.jsonOpSync("op_sleep_sync", { millis });
+  }
+
   // Derived from https://github.com/vadimg/js_bintrees. MIT Licensed.
 
   class RBNode {
@@ -545,5 +549,6 @@
     opStopGlobalTimer,
     opStartGlobalTimer,
     opNow,
+    sleepSync,
   };
 })(this);
diff --git a/cli/rt/90_deno_ns.js b/cli/rt/90_deno_ns.js
index 3a41e6bd11..7e8598923a 100644
--- a/cli/rt/90_deno_ns.js
+++ b/cli/rt/90_deno_ns.js
@@ -80,6 +80,7 @@ __bootstrap.denoNs = {
   listen: __bootstrap.net.listen,
   connectTls: __bootstrap.tls.connectTls,
   listenTls: __bootstrap.tls.listenTls,
+  sleepSync: __bootstrap.timers.sleepSync,
 };
 
 __bootstrap.denoNsUnstable = {
diff --git a/cli/tests/unit/timers_test.ts b/cli/tests/unit/timers_test.ts
index b304d89466..2d69ddb446 100644
--- a/cli/tests/unit/timers_test.ts
+++ b/cli/tests/unit/timers_test.ts
@@ -380,3 +380,55 @@ unitTest(async function timerIgnoresDateOverride(): Promise<void> {
   }
   assertEquals(hasThrown, 1);
 });
+
+unitTest({ perms: { hrtime: true } }, function sleepSync(): void {
+  const start = performance.now();
+  Deno.sleepSync(10);
+  const after = performance.now();
+  assert(after - start >= 10);
+});
+
+unitTest(
+  { perms: { hrtime: true } },
+  async function sleepSyncShorterPromise(): Promise<void> {
+    const perf = performance;
+    const short = 5;
+    const long = 10;
+
+    const start = perf.now();
+    const p = sleepAsync(short).then(() => {
+      const after = perf.now();
+      // pending promises should resolve after the main thread comes out of sleep
+      assert(after - start >= long);
+    });
+    Deno.sleepSync(long);
+
+    await p;
+  },
+);
+
+unitTest(
+  { perms: { hrtime: true } },
+  async function sleepSyncLongerPromise(): Promise<void> {
+    const perf = performance;
+    const short = 5;
+    const long = 10;
+
+    const start = perf.now();
+    const p = sleepAsync(long).then(() => {
+      const after = perf.now();
+      // sleeping for less than the duration of a promise should have no impact
+      // on the resolution of that promise
+      assert(after - start >= long);
+    });
+    Deno.sleepSync(short);
+
+    await p;
+  },
+);
+
+function sleepAsync(delay: number): Promise<void> {
+  return new Promise((resolve) => {
+    setTimeout(() => resolve(), delay);
+  });
+}