diff --git a/crates/script/src/prelude.js b/crates/script/src/prelude.js
index 1e30bef1..367d43cc 100644
--- a/crates/script/src/prelude.js
+++ b/crates/script/src/prelude.js
@@ -27,6 +27,25 @@ function __str(v) {
   return String(v);
 }
 
+// ExtendScript includes the ES3 String.substr(start, length) method, which Boa
+// does not currently provide. Follow its legacy length (not end-index) semantics.
+if (typeof String.prototype.substr !== "function") {
+  Object.defineProperty(String.prototype, "substr", {
+    value: function (start, length) {
+      if (this === null || this === undefined) throw new TypeError("String.substr called on null or undefined");
+      var text = String(this), n = text.length;
+      var from = Number(start);
+      if (isNaN(from)) from = 0;
+      // ToIntegerOrInfinity, then clamp positive or negative offsets.
+      from = from < 0 ? Math.max(n + Math.ceil(from), 0) : Math.min(Math.floor(from), n);
+      var count = length === undefined ? n - from : Number(length);
+      if (isNaN(count) || count <= 0) return "";
+      return text.slice(from, from + count);
+    },
+    writable: true, configurable: true
+  });
+}
+
 // ------------------------------------------------------------------ enums
 
 function __enum(name, keys, base) {
diff --git a/crates/script/src/tests.rs b/crates/script/src/tests.rs
index c0f44b4b..e918da7c 100644
--- a/crates/script/src/tests.rs
+++ b/crates/script/src/tests.rs
@@ -879,3 +879,25 @@ fn script_av_layer_transform_shortcuts_return_live_properties() {
     let restored = ok(&mut s, "app.project.item(1).layer(1).rotation.numKeys");
     assert_eq!(restored.result, json!(0));
 }
+
+#[test]
+fn extendscript_substr_keeps_start_and_length_semantics() {
+    let mut s = session();
+    let o = ok(
+        &mut s,
+        r#"[
+          "abcdef".substr(0, 2),
+          "abcdef".substr(2, 3),
+          "abcdef".substr(-2),
+          "abcdef".substr(-20, 2),
+          "abcdef".substr(2, 0),
+          "abcdef".substr(2, -3),
+          "abcdef".substr(2.9, 2.9),
+          "abcdef".substr(3),
+          "abcdef".substr(30, 2),
+          "abcdef".substr(NaN, 2),
+          "abcdef".substr(2, NaN)
+        ]"#,
+    );
+    assert_eq!(o.result, json!(["ab", "cde", "ef", "ab", "", "", "cd", "def", "", "ab", ""]));
+}