diff --git a/core/engine/src/builtins/temporal/plain_time/tests.rs b/core/engine/src/builtins/temporal/plain_time/tests.rs
index f8a62ded721..e94a155a57d 100644
--- a/core/engine/src/builtins/temporal/plain_time/tests.rs
+++ b/core/engine/src/builtins/temporal/plain_time/tests.rs
@@ -42,3 +42,88 @@ fn with_overflow_reject_throws_on_negative_components() {
         ),
     ]);
 }
+
+#[test]
+fn pt_to_locale_string_basic() {
+    run_test_actions([
+        TestAction::run(
+            "let pt = new Temporal.PlainTime(15, 23, 30, 123, 456, 789)",
+        ),
+        // toLocaleString should return a string
+        TestAction::assert("typeof pt.toLocaleString() === 'string'"),
+        // Should contain time components
+        TestAction::assert("pt.toLocaleString().includes('15')"),
+        TestAction::assert("pt.toLocaleString().includes('23')"),
+        TestAction::assert("pt.toLocaleString().includes('30')"),
+    ]);
+}
+
+#[test]
+fn pt_to_locale_string_with_midnight() {
+    run_test_actions([
+        TestAction::run(
+            "let pt = new Temporal.PlainTime(0, 0, 0)",
+        ),
+        // Midnight should work
+        TestAction::assert("typeof pt.toLocaleString() === 'string'"),
+        TestAction::assert("pt.toLocaleString().includes('00')"),
+    ]);
+}
+
+#[test]
+fn pt_to_locale_string_with_noon() {
+    run_test_actions([
+        TestAction::run(
+            "let pt = new Temporal.PlainTime(12, 0, 0)",
+        ),
+        // Noon should work
+        TestAction::assert("typeof pt.toLocaleString() === 'string'"),
+        TestAction::assert("pt.toLocaleString().includes('12')"),
+    ]);
+}
+
+#[test]
+fn pt_to_locale_string_with_locales() {
+    run_test_actions([
+        TestAction::run(
+            "let pt = new Temporal.PlainTime(15, 23, 30)",
+        ),
+        // Should accept locales parameter (even if not fully implemented yet)
+        TestAction::assert("typeof pt.toLocaleString('en-US') === 'string'"),
+        TestAction::assert("typeof pt.toLocaleString(['en-US', 'fr-FR']) === 'string'"),
+    ]);
+}
+
+#[test]
+fn pt_to_locale_string_with_options() {
+    run_test_actions([
+        TestAction::run(
+            "let pt = new Temporal.PlainTime(15, 23, 30)",
+        ),
+        // Should accept options parameter (even if not fully implemented yet)
+        TestAction::assert("typeof pt.toLocaleString(undefined, {}) === 'string'"),
+    ]);
+}
+
+#[test]
+fn pt_to_locale_string_different_times() {
+    run_test_actions([
+        TestAction::run(
+            "let pt1 = new Temporal.PlainTime(9, 30)",
+        ),
+        TestAction::run(
+            "let pt2 = new Temporal.PlainTime(14, 45, 30)",
+        ),
+        TestAction::run(
+            "let pt3 = new Temporal.PlainTime(23, 59, 59, 999)",
+        ),
+        // All should return strings
+        TestAction::assert("typeof pt1.toLocaleString() === 'string'"),
+        TestAction::assert("typeof pt2.toLocaleString() === 'string'"),
+        TestAction::assert("typeof pt3.toLocaleString() === 'string'"),
+        // Should contain correct hour values
+        TestAction::assert("pt1.toLocaleString().includes('09')"),
+        TestAction::assert("pt2.toLocaleString().includes('14')"),
+        TestAction::assert("pt3.toLocaleString().includes('23')"),
+    ]);
+}