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..70e60246915 100644
--- a/core/engine/src/builtins/temporal/plain_month_day/mod.rs
+++ b/core/engine/src/builtins/temporal/plain_month_day/mod.rs
@@ -627,3 +627,6 @@ fn to_temporal_month_day(
     // 16. Return ! CreateTemporalMonthDay(isoDate, calendar).
     Ok(InnerMonthDay::from_parsed(parse_record)?)
 }
+
+#[cfg(test)]
+mod tests;
diff --git a/core/engine/src/builtins/temporal/plain_month_day/tests.rs b/core/engine/src/builtins/temporal/plain_month_day/tests.rs
new file mode 100644
index 00000000000..36b6cebca7a
--- /dev/null
+++ b/core/engine/src/builtins/temporal/plain_month_day/tests.rs
@@ -0,0 +1,86 @@
+use crate::{TestAction, run_test_actions};
+
+#[test]
+fn pmd_basic_construction() {
+    run_test_actions([
+        TestAction::run(
+            "let pmd = new Temporal.PlainMonthDay(12, 25)",
+        ),
+        TestAction::assert("pmd.monthCode === 'M12'"),
+        TestAction::assert("pmd.day === 25"),
+    ]);
+}
+
+#[test]
+fn pmd_to_locale_string_basic() {
+    run_test_actions([
+        TestAction::run(
+            "let pmd = new Temporal.PlainMonthDay(12, 25)",
+        ),
+        // toLocaleString should return a string
+        TestAction::assert("typeof pmd.toLocaleString() === 'string'"),
+        // Should contain month and day components
+        TestAction::assert("pmd.toLocaleString().includes('12')"),
+        TestAction::assert("pmd.toLocaleString().includes('25')"),
+    ]);
+}
+
+#[test]
+fn pmd_to_locale_string_with_calendar() {
+    run_test_actions([
+        TestAction::run(
+            "let pmd = new Temporal.PlainMonthDay(12, 25, 'iso8601')",
+        ),
+        // Should accept calendar parameter
+        TestAction::assert("typeof pmd.toLocaleString() === 'string'"),
+        // Should contain month and day information
+        TestAction::assert("pmd.toLocaleString().includes('12')"),
+        TestAction::assert("pmd.toLocaleString().includes('25')"),
+    ]);
+}
+
+#[test]
+fn pmd_to_locale_string_with_locales() {
+    run_test_actions([
+        TestAction::run(
+            "let pmd = new Temporal.PlainMonthDay(12, 25)",
+        ),
+        // Should accept locales parameter (even if not fully implemented yet)
+        TestAction::assert("typeof pmd.toLocaleString('en-US') === 'string'"),
+        TestAction::assert("typeof pmd.toLocaleString(['en-US', 'fr-FR']) === 'string'"),
+    ]);
+}
+
+#[test]
+fn pmd_to_locale_string_with_options() {
+    run_test_actions([
+        TestAction::run(
+            "let pmd = new Temporal.PlainMonthDay(12, 25)",
+        ),
+        // Should accept options parameter (even if not fully implemented yet)
+        TestAction::assert("typeof pmd.toLocaleString(undefined, {}) === 'string'"),
+    ]);
+}
+
+#[test]
+fn pmd_to_locale_string_different_dates() {
+    run_test_actions([
+        TestAction::run(
+            "let pmd1 = new Temporal.PlainMonthDay(1, 1)",
+        ),
+        TestAction::run(
+            "let pmd2 = new Temporal.PlainMonthDay(7, 4)",
+        ),
+        TestAction::run(
+            "let pmd3 = new Temporal.PlainMonthDay(10, 31)",
+        ),
+        // All should return strings
+        TestAction::assert("typeof pmd1.toLocaleString() === 'string'"),
+        TestAction::assert("typeof pmd2.toLocaleString() === 'string'"),
+        TestAction::assert("typeof pmd3.toLocaleString() === 'string'"),
+        // Should contain correct values
+        TestAction::assert("pmd1.toLocaleString().includes('01')"),
+        TestAction::assert("pmd2.toLocaleString().includes('07')"),
+        TestAction::assert("pmd3.toLocaleString().includes('10')"),
+    ]);
+}