diff --git a/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp b/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp
index 167c3eb667..3d93baa7ae 100644
--- a/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp
+++ b/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp
@@ -1334,6 +1334,25 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> writable_stream_default_w
     return writable_stream_close(*stream);
 }
 
+// https://streams.spec.whatwg.org/#writable-stream-default-writer-ensure-closed-promise-rejected
+void writable_stream_default_writer_ensure_closed_promise_rejected(WritableStreamDefaultWriter& writer, JS::Value error)
+{
+    auto& realm = writer.realm();
+
+    // 1. If writer.[[closedPromise]].[[PromiseState]] is "pending", reject writer.[[closedPromise]] with error.
+    auto& closed_promise = verify_cast<JS::Promise>(*writer.closed_promise()->promise());
+    if (closed_promise.state() == JS::Promise::State::Pending) {
+        WebIDL::reject_promise(realm, *writer.closed_promise(), error);
+    }
+    // 2. Otherwise, set writer.[[closedPromise]] to a promise rejected with error.
+    else {
+        writer.set_closed_promise(WebIDL::create_rejected_promise(realm, error));
+    }
+
+    // 3. Set writer.[[closedPromise]].[[PromiseIsHandled]] to true.
+    WebIDL::mark_promise_as_handled(*writer.closed_promise());
+}
+
 // https://streams.spec.whatwg.org/#writable-stream-default-writer-ensure-ready-promise-rejected
 void writable_stream_default_writer_ensure_ready_promise_rejected(WritableStreamDefaultWriter& writer, JS::Value error)
 {
@@ -1374,6 +1393,36 @@ Optional<double> writable_stream_default_writer_get_desired_size(WritableStreamD
     return writable_stream_default_controller_get_desired_size(*stream->controller());
 }
 
+// https://streams.spec.whatwg.org/#writable-stream-default-writer-release
+WebIDL::ExceptionOr<void> writable_stream_default_writer_release(WritableStreamDefaultWriter& writer)
+{
+    // 1. Let stream be writer.[[stream]].
+    auto stream = writer.stream();
+
+    // 2. Assert: stream is not undefined.
+    VERIFY(stream);
+
+    // 3. Assert: stream.[[writer]] is writer.
+    VERIFY(stream->writer().ptr() == &writer);
+
+    // 4. Let releasedError be a new TypeError.
+    auto released_error = MUST_OR_THROW_OOM(JS::TypeError::create(writer.realm(), "Writer's stream lock has been released"sv));
+
+    // 5. Perform ! WritableStreamDefaultWriterEnsureReadyPromiseRejected(writer, releasedError).
+    writable_stream_default_writer_ensure_ready_promise_rejected(writer, released_error);
+
+    // 6. Perform ! WritableStreamDefaultWriterEnsureClosedPromiseRejected(writer, releasedError).
+    writable_stream_default_writer_ensure_closed_promise_rejected(writer, released_error);
+
+    // 7. Set stream.[[writer]] to undefined.
+    stream->set_writer({});
+
+    // 8. Set writer.[[stream]] to undefined.
+    writer.set_stream({});
+
+    return {};
+}
+
 // https://streams.spec.whatwg.org/#writable-stream-default-controller-advance-queue-if-needed
 WebIDL::ExceptionOr<void> writable_stream_default_controller_advance_queue_if_needed(WritableStreamDefaultController& controller)
 {
diff --git a/Userland/Libraries/LibWeb/Streams/AbstractOperations.h b/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
index a104192b6c..9e198f0751 100644
--- a/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
+++ b/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
@@ -75,8 +75,10 @@ void writable_stream_update_backpressure(WritableStream&, bool backpressure);
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> writable_stream_default_writer_abort(WritableStreamDefaultWriter&, JS::Value reason);
 WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> writable_stream_default_writer_close(WritableStreamDefaultWriter&);
+void writable_stream_default_writer_ensure_closed_promise_rejected(WritableStreamDefaultWriter&, JS::Value error);
 void writable_stream_default_writer_ensure_ready_promise_rejected(WritableStreamDefaultWriter&, JS::Value error);
 Optional<double> writable_stream_default_writer_get_desired_size(WritableStreamDefaultWriter const&);
+WebIDL::ExceptionOr<void> writable_stream_default_writer_release(WritableStreamDefaultWriter&);
 
 WebIDL::ExceptionOr<void> writable_stream_default_controller_advance_queue_if_needed(WritableStreamDefaultController&);
 void writable_stream_default_controller_clear_algorithms(WritableStreamDefaultController&);
diff --git a/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.cpp b/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.cpp
index de9c927ad0..ae98f43037 100644
--- a/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.cpp
+++ b/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.cpp
@@ -87,6 +87,22 @@ WebIDL::ExceptionOr<JS::GCPtr<JS::Object>> WritableStreamDefaultWriter::close()
     return TRY(writable_stream_default_writer_close(*this))->promise();
 }
 
+// https://streams.spec.whatwg.org/#default-writer-release-lock
+WebIDL::ExceptionOr<void> WritableStreamDefaultWriter::release_lock()
+{
+    // 1. Let stream be this.[[stream]].
+
+    // 2. If stream is undefined, return.
+    if (!m_stream)
+        return {};
+
+    // 3. Assert: stream.[[writer]] is not undefined.
+    VERIFY(m_stream->writer());
+
+    // 4. Perform ! WritableStreamDefaultWriterRelease(this).
+    return writable_stream_default_writer_release(*this);
+}
+
 WritableStreamDefaultWriter::WritableStreamDefaultWriter(JS::Realm& realm)
     : Bindings::PlatformObject(realm)
 {
diff --git a/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.h b/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.h
index e66620fae6..dbd725051b 100644
--- a/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.h
+++ b/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.h
@@ -29,6 +29,7 @@ public:
     JS::GCPtr<JS::Object> ready();
     WebIDL::ExceptionOr<JS::GCPtr<JS::Object>> abort(JS::Value reason);
     WebIDL::ExceptionOr<JS::GCPtr<JS::Object>> close();
+    WebIDL::ExceptionOr<void> release_lock();
 
     JS::GCPtr<WebIDL::Promise> closed_promise() { return m_closed_promise; }
     void set_closed_promise(JS::GCPtr<WebIDL::Promise> value) { m_closed_promise = value; }
diff --git a/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.idl b/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.idl
index f1976344d2..25092d4070 100644
--- a/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.idl
+++ b/Userland/Libraries/LibWeb/Streams/WritableStreamDefaultWriter.idl
@@ -10,8 +10,8 @@ interface WritableStreamDefaultWriter {
 
     Promise<undefined> abort(optional any reason);
     Promise<undefined> close();
+    undefined releaseLock();
 
     // FIXME:
-    // undefined releaseLock();
     // Promise<undefined> write(optional any chunk);
 };