diff --git a/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.cpp b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.cpp
index 84b8fd74f6..d22a51f320 100644
--- a/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.cpp
+++ b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.cpp
@@ -43,4 +43,13 @@ Optional<double> TransformStreamDefaultController::desired_size()
     return readable_stream_default_controller_get_desired_size(*readable_controller);
 }
 
+// https://streams.spec.whatwg.org/#ts-default-controller-error
+WebIDL::ExceptionOr<void> TransformStreamDefaultController::error(Optional<JS::Value> reason)
+{
+    // 1. Perform ? TransformStreamDefaultControllerError(this, e).
+    TRY(transform_stream_default_controller_error(*this, reason.has_value() ? reason.value() : JS::js_undefined()));
+
+    return {};
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.h b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.h
index 8854941a4d..5d14122b44 100644
--- a/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.h
+++ b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.h
@@ -19,6 +19,7 @@ public:
     virtual ~TransformStreamDefaultController() override;
 
     Optional<double> desired_size();
+    WebIDL::ExceptionOr<void> error(Optional<JS::Value> reason = {});
 
     auto& flush_algorithm() { return m_flush_algorithm; }
     void set_flush_algorithm(Optional<FlushAlgorithm>&& value) { m_flush_algorithm = move(value); }
diff --git a/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.idl b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.idl
index fefd284d54..9b2859062e 100644
--- a/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.idl
+++ b/Userland/Libraries/LibWeb/Streams/TransformStreamDefaultController.idl
@@ -4,6 +4,6 @@ interface TransformStreamDefaultController {
     readonly attribute unrestricted double? desiredSize;
 
     // FIXME: undefined enqueue(optional any chunk);
-    // FIXME: undefined error(optional any reason);
+    undefined error(optional any reason);
     // FIXME: undefined terminate();
 };