diff --git a/cli/js/deno_unstable.ts b/cli/js/deno_unstable.ts
index 192137e1f6..991df99553 100644
--- a/cli/js/deno_unstable.ts
+++ b/cli/js/deno_unstable.ts
@@ -12,6 +12,7 @@ export { applySourceMap, formatDiagnostics } from "./ops/errors.ts";
 export { signal, signals, Signal, SignalStream } from "./signals.ts";
 export { setRaw } from "./ops/tty.ts";
 export { utimeSync, utime } from "./ops/fs/utime.ts";
+export { ftruncateSync, ftruncate } from "./ops/fs/truncate.ts";
 export { ShutdownMode, shutdown } from "./net.ts";
 export { listen, listenDatagram, connect } from "./net_unstable.ts";
 export { startTls } from "./tls.ts";
diff --git a/cli/js/lib.deno.unstable.d.ts b/cli/js/lib.deno.unstable.d.ts
index 0a80eb2c64..dd8de2eb62 100644
--- a/cli/js/lib.deno.unstable.d.ts
+++ b/cli/js/lib.deno.unstable.d.ts
@@ -1248,4 +1248,43 @@ declare namespace Deno {
 
   /** **UNSTABLE**: The URL of the file that was originally executed from the command-line. */
   export const mainModule: string;
+
+  /** Synchronously truncates or extends the specified file stream, to reach the
+   * specified `len`.  If `len` is not specified then the entire file contents
+   * are truncated.
+   *
+   * ```ts
+   * // truncate the entire file
+   * const file = Deno.open("my_file.txt", { read: true, write: true, truncate: true, create: true });
+   * Deno.ftruncateSync(file.rid);
+   *
+   * // truncate part of the file
+   * const file = Deno.open("my_file.txt", { read: true, write: true, create: true });
+   * Deno.write(file.rid, new TextEncoder().encode("Hello World"));
+   * Deno.ftruncateSync(file.rid, 7);
+   * const data = new Uint8Array(32);
+   * Deno.readSync(file.rid, data);
+   * console.log(new TextDecoder().decode(data)); // Hello W
+   * ```
+   */
+  export function ftruncateSync(rid: number, len?: number): void;
+
+  /** Truncates or extends the specified file stream, to reach the specified `len`. If
+   * `len` is not specified then the entire file contents are truncated.
+   *
+   * ```ts
+   * // truncate the entire file
+   * const file = Deno.open("my_file.txt", { read: true, write: true, create: true });
+   * await Deno.ftruncate(file.rid);
+   *
+   * // truncate part of the file
+   * const file = Deno.open("my_file.txt", { read: true, write: true, create: true });
+   * await Deno.write(file.rid, new TextEncoder().encode("Hello World"));
+   * await Deno.ftruncate(file.rid, 7);
+   * const data = new Uint8Array(32);
+   * await Deno.read(file.rid, data);
+   * console.log(new TextDecoder().decode(data)); // Hello W
+   * ```
+   */
+  export function ftruncate(rid: number, len?: number): Promise<void>;
 }
diff --git a/cli/js/ops/fs/truncate.ts b/cli/js/ops/fs/truncate.ts
index 861e843f86..2b805e5ac7 100644
--- a/cli/js/ops/fs/truncate.ts
+++ b/cli/js/ops/fs/truncate.ts
@@ -13,6 +13,14 @@ function coerceLen(len?: number): number {
   return len;
 }
 
+export function ftruncateSync(rid: number, len?: number): void {
+  sendSync("op_ftruncate", { rid, len: coerceLen(len) });
+}
+
+export async function ftruncate(rid: number, len?: number): Promise<void> {
+  await sendAsync("op_ftruncate", { rid, len: coerceLen(len) });
+}
+
 export function truncateSync(path: string, len?: number): void {
   sendSync("op_truncate", { path, len: coerceLen(len) });
 }
diff --git a/cli/ops/fs.rs b/cli/ops/fs.rs
index 63b3ad7f56..9ca4b31aeb 100644
--- a/cli/ops/fs.rs
+++ b/cli/ops/fs.rs
@@ -36,6 +36,7 @@ pub fn init(i: &mut CoreIsolate, s: &State) {
   i.register_op("op_link", s.stateful_json_op(op_link));
   i.register_op("op_symlink", s.stateful_json_op(op_symlink));
   i.register_op("op_read_link", s.stateful_json_op(op_read_link));
+  i.register_op("op_ftruncate", s.stateful_json_op2(op_ftruncate));
   i.register_op("op_truncate", s.stateful_json_op(op_truncate));
   i.register_op("op_make_temp_dir", s.stateful_json_op(op_make_temp_dir));
   i.register_op("op_make_temp_file", s.stateful_json_op(op_make_temp_file));
@@ -783,6 +784,52 @@ fn op_read_link(
   })
 }
 
+#[derive(Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct FtruncateArgs {
+  promise_id: Option<u64>,
+  rid: i32,
+  len: i32,
+}
+
+fn op_ftruncate(
+  isolate_state: &mut CoreIsolateState,
+  state: &State,
+  args: Value,
+  _zero_copy: &mut [ZeroCopyBuf],
+) -> Result<JsonOp, OpError> {
+  state.check_unstable("Deno.ftruncate");
+  let args: FtruncateArgs = serde_json::from_value(args)?;
+  let rid = args.rid as u32;
+  let len = args.len as u64;
+
+  let resource_table = isolate_state.resource_table.clone();
+  let is_sync = args.promise_id.is_none();
+
+  if is_sync {
+    let mut resource_table = resource_table.borrow_mut();
+    std_file_resource(&mut resource_table, rid, |r| match r {
+      Ok(std_file) => std_file.set_len(len).map_err(OpError::from),
+      Err(_) => Err(OpError::type_error(
+        "cannot truncate this type of resource".to_string(),
+      )),
+    })?;
+    Ok(JsonOp::Sync(json!({})))
+  } else {
+    let fut = async move {
+      let mut resource_table = resource_table.borrow_mut();
+      std_file_resource(&mut resource_table, rid, |r| match r {
+        Ok(std_file) => std_file.set_len(len).map_err(OpError::from),
+        Err(_) => Err(OpError::type_error(
+          "cannot truncate this type of resource".to_string(),
+        )),
+      })?;
+      Ok(json!({}))
+    };
+    Ok(JsonOp::Async(fut.boxed_local()))
+  }
+}
+
 #[derive(Deserialize)]
 #[serde(rename_all = "camelCase")]
 struct TruncateArgs {
diff --git a/cli/tests/unit/truncate_test.ts b/cli/tests/unit/truncate_test.ts
index 0148157026..82f8f03eb3 100644
--- a/cli/tests/unit/truncate_test.ts
+++ b/cli/tests/unit/truncate_test.ts
@@ -1,6 +1,50 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 import { unitTest, assertEquals, assert } from "./test_util.ts";
 
+unitTest(
+  { perms: { read: true, write: true } },
+  function ftruncateSyncSuccess(): void {
+    const filename = Deno.makeTempDirSync() + "/test_ftruncateSync.txt";
+    const file = Deno.openSync(filename, {
+      create: true,
+      read: true,
+      write: true,
+    });
+
+    Deno.ftruncateSync(file.rid, 20);
+    assertEquals(Deno.readFileSync(filename).byteLength, 20);
+    Deno.ftruncateSync(file.rid, 5);
+    assertEquals(Deno.readFileSync(filename).byteLength, 5);
+    Deno.ftruncateSync(file.rid, -5);
+    assertEquals(Deno.readFileSync(filename).byteLength, 0);
+
+    Deno.close(file.rid);
+    Deno.removeSync(filename);
+  }
+);
+
+unitTest(
+  { perms: { read: true, write: true } },
+  async function ftruncateSuccess(): Promise<void> {
+    const filename = Deno.makeTempDirSync() + "/test_ftruncate.txt";
+    const file = await Deno.open(filename, {
+      create: true,
+      read: true,
+      write: true,
+    });
+
+    await Deno.ftruncate(file.rid, 20);
+    assertEquals((await Deno.readFile(filename)).byteLength, 20);
+    await Deno.ftruncate(file.rid, 5);
+    assertEquals((await Deno.readFile(filename)).byteLength, 5);
+    await Deno.ftruncate(file.rid, -5);
+    assertEquals((await Deno.readFile(filename)).byteLength, 0);
+
+    Deno.close(file.rid);
+    await Deno.remove(filename);
+  }
+);
+
 unitTest(
   { perms: { read: true, write: true } },
   function truncateSyncSuccess(): void {