diff --git a/Userland/Libraries/LibWeb/CMakeLists.txt b/Userland/Libraries/LibWeb/CMakeLists.txt
index 89f90999b8..2ca8ccccf3 100644
--- a/Userland/Libraries/LibWeb/CMakeLists.txt
+++ b/Userland/Libraries/LibWeb/CMakeLists.txt
@@ -515,6 +515,7 @@ set(SOURCES
     Streams/ReadableStreamGenericReader.cpp
     Streams/Transformer.cpp
     Streams/TransformStream.cpp
+    Streams/TransformStreamDefaultController.cpp
     Streams/UnderlyingSink.cpp
     Streams/UnderlyingSource.cpp
     Streams/WritableStream.cpp
diff --git a/Userland/Libraries/LibWeb/Streams/AbstractOperations.h b/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
index 79ecb5eb40..bebeee04b8 100644
--- a/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
+++ b/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
@@ -24,6 +24,8 @@ using StartAlgorithm = JS::SafeFunction<WebIDL::ExceptionOr<JS::Value>()>;
 using AbortAlgorithm = JS::SafeFunction<WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>>(JS::Value)>;
 using CloseAlgorithm = JS::SafeFunction<WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>>()>;
 using WriteAlgorithm = JS::SafeFunction<WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>>(JS::Value)>;
+using FlushAlgorithm = JS::SafeFunction<WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>>()>;
+using TransformAlgorithm = JS::SafeFunction<WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>>(JS::Value)>;
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<ReadableStreamDefaultReader>> acquire_readable_stream_default_reader(ReadableStream&);
 WebIDL::ExceptionOr<JS::NonnullGCPtr<ReadableStreamBYOBReader>> acquire_readable_stream_byob_reader(ReadableStream&);
diff --git a/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.cpp b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.cpp
new file mode 100644
index 0000000000..84b8fd74f6
--- /dev/null
+++ b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.cpp
@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2023, Kenneth Myhra <kennethmyhra@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/Streams/TransformStream.h>
+#include <LibWeb/Streams/TransformStreamDefaultController.h>
+
+namespace Web::Streams {
+
+TransformStreamDefaultController::TransformStreamDefaultController(JS::Realm& realm)
+    : Bindings::PlatformObject(realm)
+{
+}
+
+TransformStreamDefaultController::~TransformStreamDefaultController() = default;
+
+JS::ThrowCompletionOr<void> TransformStreamDefaultController::initialize(JS::Realm& realm)
+{
+    MUST_OR_THROW_OOM(Base::initialize(realm));
+    set_prototype(&Bindings::ensure_web_prototype<Bindings::TransformStreamDefaultControllerPrototype>(realm, "TransformStreamDefaultController"));
+
+    return {};
+}
+
+void TransformStreamDefaultController::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_stream);
+}
+
+// https://streams.spec.whatwg.org/#ts-default-controller-desired-size
+Optional<double> TransformStreamDefaultController::desired_size()
+{
+    VERIFY(stream()->readable()->controller().has_value() && stream()->readable()->controller()->has<JS::NonnullGCPtr<ReadableStreamDefaultController>>());
+
+    // 1. Let readableController be this.[[stream]].[[readable]].[[controller]].
+    auto readable_controller = stream()->readable()->controller()->get<JS::NonnullGCPtr<ReadableStreamDefaultController>>();
+
+    // 2. Return ! ReadableStreamDefaultControllerGetDesiredSize(readableController).
+    return readable_stream_default_controller_get_desired_size(*readable_controller);
+}
+
+}
diff --git a/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.h b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.h
new file mode 100644
index 0000000000..8854941a4d
--- /dev/null
+++ b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.h
@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2023, Kenneth Myhra <kennethmyhra@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Bindings/PlatformObject.h>
+#include <LibWeb/Streams/AbstractOperations.h>
+
+namespace Web::Streams {
+
+class TransformStreamDefaultController : public Bindings::PlatformObject {
+    WEB_PLATFORM_OBJECT(TransformStreamDefaultController, Bindings::PlatformObject);
+
+public:
+    explicit TransformStreamDefaultController(JS::Realm&);
+    virtual ~TransformStreamDefaultController() override;
+
+    Optional<double> desired_size();
+
+    auto& flush_algorithm() { return m_flush_algorithm; }
+    void set_flush_algorithm(Optional<FlushAlgorithm>&& value) { m_flush_algorithm = move(value); }
+
+    auto& transform_algorithm() { return m_transform_algorithm; }
+    void set_transform_algorithm(Optional<TransformAlgorithm>&& value) { m_transform_algorithm = move(value); }
+
+    JS::GCPtr<TransformStream> stream() { return m_stream; }
+    void set_stream(JS::GCPtr<TransformStream> stream) { m_stream = stream; }
+
+private:
+    virtual JS::ThrowCompletionOr<void> initialize(JS::Realm&) override;
+
+    virtual void visit_edges(Cell::Visitor&) override;
+
+    // https://streams.spec.whatwg.org/#transformstreamdefaultcontroller-flushalgorithm
+    Optional<FlushAlgorithm> m_flush_algorithm;
+
+    // https://streams.spec.whatwg.org/#transformstreamdefaultcontroller-transformalgorithm
+    Optional<TransformAlgorithm> m_transform_algorithm;
+
+    // https://streams.spec.whatwg.org/#transformstreamdefaultcontroller-stream
+    JS::GCPtr<TransformStream> m_stream;
+};
+
+}
diff --git a/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.idl b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.idl
new file mode 100644
index 0000000000..fefd284d54
--- /dev/null
+++ b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.idl
@@ -0,0 +1,9 @@
+// https://streams.spec.whatwg.org/#transformstreamdefaultcontroller
+[Exposed=*]
+interface TransformStreamDefaultController {
+    readonly attribute unrestricted double? desiredSize;
+
+    // FIXME: undefined enqueue(optional any chunk);
+    // FIXME: undefined error(optional any reason);
+    // FIXME: undefined terminate();
+};
diff --git a/Userland/Libraries/LibWeb/idl_files.cmake b/Userland/Libraries/LibWeb/idl_files.cmake
index b9f8bd2897..bb85128758 100644
--- a/Userland/Libraries/LibWeb/idl_files.cmake
+++ b/Userland/Libraries/LibWeb/idl_files.cmake
@@ -197,6 +197,7 @@ libweb_js_bindings(Streams/ReadableStreamBYOBRequest)
 libweb_js_bindings(Streams/ReadableStreamDefaultController)
 libweb_js_bindings(Streams/ReadableStreamDefaultReader)
 libweb_js_bindings(Streams/TransformStream)
+libweb_js_bindings(Streams/TransformStreamDefaultController)
 libweb_js_bindings(Streams/WritableStream)
 libweb_js_bindings(Streams/WritableStreamDefaultWriter)
 libweb_js_bindings(SVG/SVGAnimatedLength)