diff --git a/core/engine/src/builtins/intl/date_time_format/mod.rs b/core/engine/src/builtins/intl/date_time_format/mod.rs
index cc692602c79..f0b0a1ac5f6 100644
--- a/core/engine/src/builtins/intl/date_time_format/mod.rs
+++ b/core/engine/src/builtins/intl/date_time_format/mod.rs
@@ -1053,3 +1053,70 @@ pub(crate) fn format_date_time_locale(
     let result = format_timestamp_with_dtf(&dtf, x, context)?;
     Ok(JsValue::from(result))
 }
+/// Formats a `Temporal.PlainMonthDay` using locale-sensitive formatting.
+///
+/// Used by `Temporal.PlainMonthDay.prototype.toLocaleString`.
+/// Unlike `format_date_time_locale`, this does not use a Unix timestamp —
+/// `PlainMonthDay` has no year or timezone, so we build an ICU4X `Date<Iso>`
+/// directly using the Temporal spec's reference year (1972).
+pub(crate) fn format_plain_month_day_locale(
+    locales: &JsValue,
+    options: &JsValue,
+    month: u8,
+    day: u8,
+    context: &mut Context,
+) -> JsResult<JsValue> {
+    let options_obj = coerce_options_to_object(options, context)?;
+
+    // Per ECMA-402 ToDateTimeOptions with type=date, defaults=date:
+    // if neither dateStyle nor individual month/day fields are set, default to
+    // { month: "long", day: "numeric" } — which is the most natural for a month-day.
+    let has_date_style =
+        get_option::<DateStyle>(&options_obj, js_string!("dateStyle"), context)?.is_some();
+    let has_month = options_obj
+        .has_own_property(js_string!("month"), context)?;
+    let has_day = options_obj
+        .has_own_property(js_string!("day"), context)?;
+
+    if !has_date_style && !has_month && !has_day {
+        options_obj.create_data_property_or_throw(
+            js_string!("month"),
+            JsValue::from(js_string!("long")),
+            context,
+        )?;
+        options_obj.create_data_property_or_throw(
+            js_string!("day"),
+            JsValue::from(js_string!("numeric")),
+            context,
+        )?;
+    }
+
+    let options_value = JsValue::from(options_obj);
+    let dtf = create_date_time_format(
+        locales,
+        &options_value,
+        FormatType::Date,
+        FormatDefaults::Date,
+        context,
+    )?;
+
+    // Use 1972 as the reference year — same year the Temporal spec uses for
+    // PlainMonthDay's internal ISODate when calendar is iso8601.
+    let date = Date::try_new_iso(1972, month, day)
+        .map_err(|e| JsNativeError::range().with_message(e.to_string()))?;
+    let time = Time::try_new(12, 0, 0, 0)
+        .map_err(|e| JsNativeError::range().with_message(e.to_string()))?;
+
+    // We need a ZonedDateTime for the formatter — use UTC offset zero.
+    let tz_info = FormatTimeZone::UtcOffset(UtcOffset::zero());
+    let dt = DateTime { date, time };
+    let tz_info_at_time = tz_info.to_time_zone_info().at_date_time_iso(dt);
+    let zdt = ZonedDateTime {
+        date,
+        time,
+        zone: tz_info_at_time,
+    };
+
+    let result = dtf.formatter.format(&zdt).to_string();
+    Ok(JsValue::from(JsString::from(result)))
+}
\ No newline at end of file
diff --git a/core/engine/src/builtins/temporal/plain_month_day/mod.rs b/core/engine/src/builtins/temporal/plain_month_day/mod.rs
index dde472d2a2b..3e449f5cb31 100644
--- a/core/engine/src/builtins/temporal/plain_month_day/mod.rs
+++ b/core/engine/src/builtins/temporal/plain_month_day/mod.rs
@@ -374,22 +374,42 @@ impl PlainMonthDay {
     ///
     /// [spec]: https://tc39.es/proposal-temporal/#sec-temporal.plainmonthday.tolocalestring
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal/PlainMonthDay/toLocaleString
-    pub(crate) fn to_locale_string(
-        this: &JsValue,
-        _: &[JsValue],
-        _: &mut Context,
-    ) -> JsResult<JsValue> {
-        // TODO: Update for ECMA-402 compliance
-        let object = this.as_object();
-        let month_day = object
-            .as_ref()
-            .and_then(JsObject::downcast_ref::<Self>)
-            .ok_or_else(|| {
-                JsNativeError::typ().with_message("this value must be a PlainMonthDay object.")
-            })?;
-
-        Ok(JsString::from(month_day.inner.to_string()).into())
-    }
+/// 10.3.9 `Temporal.PlainMonthDay.prototype.toLocaleString ( [ locales [ , options ] ] )`
+///
+/// More information:
+///
+/// - [ECMAScript Temporal proposal][spec]
+/// - [MDN reference][mdn]
+///
+/// [spec]: https://tc39.es/proposal-temporal/#sec-temporal.plainmonthday.tolocalestring
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal/PlainMonthDay/toLocaleString
+pub(crate) fn to_locale_string(
+    this: &JsValue,
+    args: &[JsValue],
+    context: &mut Context,
+) -> JsResult<JsValue> {
+    // 1. Let monthDay be the this value.
+    // 2. Perform ? RequireInternalSlot(monthDay, [[InitializedTemporalMonthDay]]).
+    let object = this.as_object();
+    let month_day = object
+        .as_ref()
+        .and_then(JsObject::downcast_ref::<Self>)
+        .ok_or_else(|| {
+            JsNativeError::typ().with_message("this value must be a PlainMonthDay object.")
+        })?;
+
+    // 3. Return ? FormatDateTimeLocaleString(monthDay, locales, options).
+    let locales = args.get_or_undefined(0);
+    let options = args.get_or_undefined(1);
+
+    crate::builtins::intl::date_time_format::format_plain_month_day_locale(
+        locales,
+        options,
+        month_day.inner.month(),
+        month_day.inner.day(),
+        context,
+    )
+}
 
     /// 10.3.10 `Temporal.PlainMonthDay.prototype.toJSON ( )`
     ///