diff --git a/Userland/Libraries/LibWeb/Streams/TransformStream.cpp b/Userland/Libraries/LibWeb/Streams/TransformStream.cpp
index 79dc4a8fad..ed05e0dbf9 100644
--- a/Userland/Libraries/LibWeb/Streams/TransformStream.cpp
+++ b/Userland/Libraries/LibWeb/Streams/TransformStream.cpp
@@ -7,8 +7,10 @@
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/Streams/AbstractOperations.h>
 #include <LibWeb/Streams/TransformStream.h>
+#include <LibWeb/Streams/TransformStreamDefaultController.h>
 #include <LibWeb/Streams/Transformer.h>
 #include <LibWeb/Streams/WritableStream.h>
+#include <LibWeb/WebIDL/AbstractOperations.h>
 #include <LibWeb/WebIDL/ExceptionOr.h>
 
 namespace Web::Streams {
@@ -16,11 +18,58 @@ namespace Web::Streams {
 // https://streams.spec.whatwg.org/#ts-construct
 WebIDL::ExceptionOr<JS::NonnullGCPtr<TransformStream>> TransformStream::construct_impl(JS::Realm& realm, Optional<JS::Handle<JS::Object>> transformer_object, QueuingStrategy const& writable_strategy, QueuingStrategy const& readable_strategy)
 {
-    (void)transformer_object;
-    (void)writable_strategy;
-    (void)readable_strategy;
+    auto& vm = realm.vm();
 
-    return MUST_OR_THROW_OOM(realm.heap().allocate<TransformStream>(realm, realm));
+    auto stream = MUST_OR_THROW_OOM(realm.heap().allocate<TransformStream>(realm, realm));
+
+    // 1. If transformer is missing, set it to null.
+    auto transformer = transformer_object.has_value() ? JS::Value { transformer_object.value().ptr() } : JS::js_null();
+
+    // 2. Let transformerDict be transformer, converted to an IDL value of type Transformer.
+    auto transformer_dict = TRY(Transformer::from_value(vm, transformer));
+
+    // 3. If transformerDict["readableType"] exists, throw a RangeError exception.
+    if (transformer_dict.readable_type.has_value())
+        return WebIDL::SimpleException { WebIDL::SimpleExceptionType::RangeError, "Invalid use of reserved key 'readableType'"sv };
+
+    // 4. If transformerDict["writableType"] exists, throw a RangeError exception.
+    if (transformer_dict.writable_type.has_value())
+        return WebIDL::SimpleException { WebIDL::SimpleExceptionType::RangeError, "Invalid use of reserved key 'writableType'"sv };
+
+    // 5. Let readableHighWaterMark be ? ExtractHighWaterMark(readableStrategy, 0).
+    auto readable_high_water_mark = TRY(extract_high_water_mark(readable_strategy, 0));
+
+    // 6. Let readableSizeAlgorithm be ! ExtractSizeAlgorithm(readableStrategy).
+    auto readable_size_algorithm = extract_size_algorithm(readable_strategy);
+
+    // 7. Let writableHighWaterMark be ? ExtractHighWaterMark(writableStrategy, 1).
+    auto writable_high_water_mark = TRY(extract_high_water_mark(writable_strategy, 1));
+
+    // 8. Let writableSizeAlgorithm be ! ExtractSizeAlgorithm(writableStrategy).
+    auto writable_size_algorithm = extract_size_algorithm(writable_strategy);
+
+    // 9. Let startPromise be a new promise.
+    auto start_promise = WebIDL::create_promise(realm);
+
+    // 10. Perform ! InitializeTransformStream(this, startPromise, writableHighWaterMark, writableSizeAlgorithm,
+    // readableHighWaterMark, readableSizeAlgorithm).
+    TRY(initialize_transform_stream(*stream, start_promise, writable_high_water_mark, move(writable_size_algorithm), readable_high_water_mark, move(readable_size_algorithm)));
+
+    // 11. Perform ? SetUpTransformStreamDefaultControllerFromTransformer(this, transformer, transformerDict).
+    TRY(set_up_transform_stream_default_controller_from_transformer(*stream, transformer, transformer_dict));
+
+    // 12. If transformerDict["start"] exists, then resolve startPromise with the result of invoking
+    // transformerDict["start"] with argument list « this.[[controller]] » and callback this value transformer.
+    if (transformer_dict.start) {
+        auto result = MUST_OR_THROW_OOM(WebIDL::invoke_callback(*transformer_dict.start, transformer, stream->controller())).release_value();
+        WebIDL::resolve_promise(realm, start_promise, result);
+    }
+    // 13. Otherwise, resolve startPromise with undefined.
+    else {
+        WebIDL::resolve_promise(realm, start_promise, JS::js_undefined());
+    }
+
+    return stream;
 }
 
 TransformStream::TransformStream(JS::Realm& realm)