diff --git a/moli-renderer-v8/src/context_bootstrap/url_search_params_runtime.rs b/moli-renderer-v8/src/context_bootstrap/url_search_params_runtime.rs
index 6235bf2619..a19aec44e2 100644
--- a/moli-renderer-v8/src/context_bootstrap/url_search_params_runtime.rs
+++ b/moli-renderer-v8/src/context_bootstrap/url_search_params_runtime.rs
@@ -1,7 +1,6 @@
 use super::form_data_runtime::{form_data_entries, form_data_is_object};
 use super::url_form::{
-    apply_url_update, callback_arg_url_like_string, callback_value_string,
-    object_prototype_matches, url_object_value,
+    apply_url_update, callback_arg_url_like_string, callback_value_string, url_object_value,
 };
 use super::*;
 use crate::util::{get_private_object, get_private_value, set_private_value};
diff --git a/moli-renderer-v8/src/context_bootstrap/url_search_params_runtime/storage.rs b/moli-renderer-v8/src/context_bootstrap/url_search_params_runtime/storage.rs
index 9313820fc0..bb3bebbb37 100644
--- a/moli-renderer-v8/src/context_bootstrap/url_search_params_runtime/storage.rs
+++ b/moli-renderer-v8/src/context_bootstrap/url_search_params_runtime/storage.rs
@@ -102,8 +102,9 @@ pub(super) fn url_search_params_is_object<'s>(
     scope: &mut v8::PinScope<'s, '_>,
     object: v8::Local<'s, v8::Object>,
 ) -> bool {
-    object_prototype_matches(scope, object, "URLSearchParams")
-        && url_search_params_pairs_array(scope, object).is_some()
+    // The internal slot is the WebIDL brand. A constructed subclass has this
+    // slot even though its direct prototype is not URLSearchParams.prototype.
+    url_search_params_pairs_array(scope, object).is_some()
 }
 
 pub(crate) fn url_search_params_request_body<'s>(
diff --git a/moli-renderer-v8/src/script_vm/tests/webidl_fetch.rs b/moli-renderer-v8/src/script_vm/tests/webidl_fetch.rs
index 8eacf6961e..f4b4c4bc5e 100644
--- a/moli-renderer-v8/src/script_vm/tests/webidl_fetch.rs
+++ b/moli-renderer-v8/src/script_vm/tests/webidl_fetch.rs
@@ -8337,6 +8337,49 @@ fn webidl_iterator_prototypes_use_v8_intrinsics_after_public_tampering() {
     assert_eq!(result, "");
 }
 
+#[test]
+fn url_search_params_subclasses_retain_their_webidl_brand() {
+    let mut vm = new_storage_test_vm("https://url-search-params-subclass.test/");
+
+    let result = vm
+        .eval(
+            r#"
+(() => {
+  class ReadonlyURLSearchParams extends URLSearchParams {
+    append() { throw new Error('readonly'); }
+    delete() { throw new Error('readonly'); }
+    set() { throw new Error('readonly'); }
+    sort() { throw new Error('readonly'); }
+  }
+
+  const params = new ReadonlyURLSearchParams('loc=fr&loc=de&empty=');
+  const seen = [];
+  params.forEach((value, name, owner) => {
+    seen.push(`${name}:${value}:${owner === params}`);
+  });
+
+  return JSON.stringify({
+    directPrototypeIsSubclass:
+      Object.getPrototypeOf(params) === ReadonlyURLSearchParams.prototype,
+    get: params.get('loc'),
+    getAll: params.getAll('loc').join(','),
+    has: params.has('empty'),
+    size: params.size,
+    entries: Array.from(params.entries()).map(pair => pair.join(':')).join(','),
+    seen,
+    serialized: params.toString()
+  });
+})()
+"#,
+        )
+        .expect("URLSearchParams subclass WebIDL operations should evaluate");
+
+    assert_eq!(
+        result,
+        r#"{"directPrototypeIsSubclass":true,"get":"fr","getAll":"fr,de","has":true,"size":3,"entries":"loc:fr,loc:de,empty:","seen":["loc:fr:true","loc:de:true","empty::true"],"serialized":"loc=fr&loc=de&empty="}"#
+    );
+}
+
 #[test]
 fn url_and_search_params_declared_slots_ignore_prototype_spoofing() {
     let mut vm = new_storage_test_vm("https://url-declared-slots.test/");