diff --git a/changelog.d/9339-array-prototype-define-property-index.md b/changelog.d/9339-array-prototype-define-property-index.md
new file mode 100644
index 0000000000..947fd5338a
--- /dev/null
+++ b/changelog.d/9339-array-prototype-define-property-index.md
@@ -0,0 +1,8 @@
+### fix(runtime): honor prototype index descriptors installed with defineProperty
+
+Indexed array assignments now observe accessors and non-writable data
+properties installed on `Array.prototype` or `Object.prototype` through
+`Object.defineProperty`, `Object.defineProperties`, or
+`Reflect.defineProperty`. Descriptor installation raises the same prototype
+invalidation latch as a plain indexed write, and the strict store fallback now
+walks the default prototype chain before creating an own element. Fixes #9249.
diff --git a/crates/perry-runtime/src/array/indexing.rs b/crates/perry-runtime/src/array/indexing.rs
index acec2003d0..f08344312a 100644
--- a/crates/perry-runtime/src/array/indexing.rs
+++ b/crates/perry-runtime/src/array/indexing.rs
@@ -1228,20 +1228,24 @@ fn js_array_set_f64_extend_strict_impl(
         return js_array_set_f64_extend(arr, index, value);
     }
 
-    // #9220: only a retargeted array with no own index pays the inherited
-    // [[Set]] walk. `array_custom_prototype` is the #9219 classification shared
-    // with reads/HasProperty and deliberately returns None for a Proxy
+    // #9220 / #9249: only a prototype-sensitive array with no own index pays
+    // the inherited [[Set]] walk. This includes both a retargeted receiver and
+    // the default chain after an index is installed on `Array.prototype` or
+    // `Object.prototype`. `array_custom_prototype` is the #9219 classification
+    // shared with reads/HasProperty and deliberately returns None for a Proxy
     // prototype, whose dedicated dispatch must remain single-shot. Existing
     // own elements have already had every applicable dense lane above; the
     // fallback still needs the ownership check for descriptor/restricted
     // shapes that correctly declined those lanes.
-    // The process latch leads for the same reason it does in the read/HasProperty
-    // twin (`generic::real_array_uses_recorded_spec_path`): recording a
-    // prototype on ANY array sets it, so a clear latch proves this array cannot
-    // have one and the side-table probe is skipped entirely.
+    //
+    // The process latches lead for the same reason they do in the
+    // read/HasProperty twin: when all are clear, no inherited indexed property
+    // can intercept this store and the side-table probe is skipped entirely.
     if !prototype_already_checked
-        && crate::object::prototype_chain::array_static_proto_recorded()
-        && unsafe { array_custom_prototype(clean).is_some() }
+        && (array_prototype_has_index_flag()
+            || object_prototype_has_index_flag()
+            || (crate::object::prototype_chain::array_static_proto_recorded()
+                && unsafe { array_custom_prototype(clean).is_some() }))
         && unsafe { !array_has_own_index(clean, index) }
     {
         return array_spec_set(clean, index, value);
diff --git a/crates/perry-runtime/src/array/mod.rs b/crates/perry-runtime/src/array/mod.rs
index 74285f8582..5fb029c72a 100644
--- a/crates/perry-runtime/src/array/mod.rs
+++ b/crates/perry-runtime/src/array/mod.rs
@@ -153,9 +153,9 @@ pub use self::indexing::{
 pub(crate) use self::indexing_support::test_keys_array_slot_fallbacks;
 pub(crate) use self::indexing_support::{
     array_proto_iterator_modified, invalidate_array_index_fast_path,
-    keys_array_len_capped_to_capacity, keys_array_slot, note_array_proto_iterator_write,
-    note_object_prototype_index_write, object_prototype_has_index_flag,
-    PERRY_ARRAY_INDEX_FAST_PATH_INVALIDATED,
+    keys_array_len_capped_to_capacity, keys_array_slot, note_array_index_write,
+    note_array_proto_iterator_write, note_object_prototype_index_write,
+    object_prototype_has_index_flag, PERRY_ARRAY_INDEX_FAST_PATH_INVALIDATED,
 };
 pub use self::is_array::js_array_is_array;
 pub(crate) use self::iter_methods::throw_reduce_of_empty;
diff --git a/crates/perry-runtime/src/object/array_object_ops.rs b/crates/perry-runtime/src/object/array_object_ops.rs
index 46ff0a1a9a..b39f4f520d 100644
--- a/crates/perry-runtime/src/object/array_object_ops.rs
+++ b/crates/perry-runtime/src/object/array_object_ops.rs
@@ -460,6 +460,13 @@ pub(crate) unsafe fn define_array_property(
     }
 
     if let Some(index) = super::canonical_array_index(key_name) {
+        // `Object.defineProperty(Array.prototype, i, descriptor)` installs an
+        // inherited index without passing through any array element-write
+        // helper. Raise the same sticky latch those helpers do so generated
+        // stores decline their own-slot fast paths and perform the inherited
+        // descriptor walk. `Object.defineProperties` and
+        // `Reflect.defineProperty` both funnel through this branch.
+        crate::array::note_array_index_write(current_arr() as usize);
         let exists = super::has_own_helpers::array_own_key_present(current_arr(), current_key());
 
         // Array exotic `[[DefineOwnProperty]]` (ECMA-262 10.4.2.1) step 3.b: a
diff --git a/crates/perry/tests/issue_9249_array_prototype_define_property.rs b/crates/perry/tests/issue_9249_array_prototype_define_property.rs
new file mode 100644
index 0000000000..7abc78182f
--- /dev/null
+++ b/crates/perry/tests/issue_9249_array_prototype_define_property.rs
@@ -0,0 +1,132 @@
+//! Regression coverage for #9249: indexed accessors installed on
+//! `Array.prototype` through the descriptor APIs must invalidate array-store
+//! fast paths just like a plain indexed assignment to the prototype does.
+
+use std::path::PathBuf;
+use std::process::Command;
+
+fn perry_bin() -> PathBuf {
+    PathBuf::from(env!("CARGO_BIN_EXE_perry"))
+}
+
+fn compile_and_run(source: &str, expected: &str, label: &str) {
+    let dir = tempfile::tempdir().expect("tempdir");
+    let entry = dir.path().join(format!("{label}.ts"));
+    let output = dir.path().join(format!("{label}_bin"));
+    std::fs::write(&entry, source).expect("write entry");
+
+    let compile = Command::new(perry_bin())
+        .current_dir(dir.path())
+        .arg("compile")
+        .arg(&entry)
+        .arg("-o")
+        .arg(&output)
+        .arg("--no-cache")
+        .output()
+        .expect("run perry compile");
+    assert!(
+        compile.status.success(),
+        "perry compile failed for {label}\nstdout:\n{}\nstderr:\n{}",
+        String::from_utf8_lossy(&compile.stdout),
+        String::from_utf8_lossy(&compile.stderr)
+    );
+
+    let run = Command::new(&output)
+        .current_dir(dir.path())
+        .output()
+        .expect("run compiled binary");
+    assert!(
+        run.status.success(),
+        "compiled binary failed for {label}\nstdout:\n{}\nstderr:\n{}",
+        String::from_utf8_lossy(&run.stdout),
+        String::from_utf8_lossy(&run.stderr)
+    );
+    assert_eq!(
+        String::from_utf8_lossy(&run.stdout),
+        expected,
+        "{label} output must match Node\nstderr:\n{}",
+        String::from_utf8_lossy(&run.stderr)
+    );
+}
+
+#[test]
+fn define_property_array_prototype_index_setter_intercepts_numeric_store() {
+    compile_and_run(
+        r#"
+let hits = 0;
+Object.defineProperty(Array.prototype, 7, {
+  set(value) { hits++; },
+  get() { return "P"; },
+  configurable: true
+});
+const nums = [1, 2, 3];
+nums[7] = 42;
+console.log(hits, nums.length, nums[7]);
+"#,
+        "1 3 P\n",
+        "define_property",
+    );
+}
+
+#[test]
+fn define_properties_array_prototype_index_setter_intercepts_boolean_store() {
+    compile_and_run(
+        r#"
+let hits = 0;
+const descriptors: any = {};
+descriptors[9] = {
+  set(value) { hits++; },
+  get() { return "P"; },
+  configurable: true
+};
+Object.defineProperties(Array.prototype, descriptors);
+const flags = [true, false];
+flags[9] = false;
+console.log(hits, flags.length, flags[9]);
+"#,
+        "1 2 P\n",
+        "define_properties",
+    );
+}
+
+#[test]
+fn define_property_object_prototype_index_setter_intercepts_array_store() {
+    compile_and_run(
+        r#"
+let hits = 0;
+Object.defineProperty(Object.prototype, 5, {
+  set(value) { hits++; },
+  get() { return "P"; },
+  configurable: true
+});
+const values = [1];
+values[5] = 99;
+console.log(hits, values.length, values[5]);
+"#,
+        "1 1 P\n",
+        "object_prototype",
+    );
+}
+
+#[test]
+fn reflect_define_property_non_writable_prototype_index_blocks_array_store() {
+    compile_and_run(
+        r#"
+Reflect.defineProperty(Array.prototype, 11, {
+  value: "P",
+  writable: false,
+  configurable: true
+});
+const values = [1];
+let result = "no error";
+try {
+  values[11] = 99;
+} catch (error) {
+  result = error.name;
+}
+console.log(result, values.length, values[11]);
+"#,
+        "TypeError 1 P\n",
+        "reflect_define_property",
+    );
+}