diff --git a/serde-generate/runtime/dart/serde/binary_deserializer.dart b/serde-generate/runtime/dart/serde/binary_deserializer.dart
index 44eeee4cc..661140272 100644
--- a/serde-generate/runtime/dart/serde/binary_deserializer.dart
+++ b/serde-generate/runtime/dart/serde/binary_deserializer.dart
@@ -79,9 +79,9 @@ abstract class BinaryDeserializer {
   }
 
   int deserializeInt64() {
-    final result = input.getInt64(_offset, Endian.little);
+    final number = _bytesToBigInt(8, signed: true);
     _offset += 8;
-    return result;
+    return number.toInt();
   }
 
   double deserializeFloat32() {
diff --git a/serde-generate/runtime/dart/serde/binary_serializer.dart b/serde-generate/runtime/dart/serde/binary_serializer.dart
index 0d2d5b89b..899c49063 100644
--- a/serde-generate/runtime/dart/serde/binary_serializer.dart
+++ b/serde-generate/runtime/dart/serde/binary_serializer.dart
@@ -101,7 +101,13 @@ abstract class BinarySerializer {
   }
 
   void serializeInt64(int value) {
-    final bdata = ByteData(8)..setInt64(0, value, Endian.little);
+    BigInt number = BigInt.from(value);
+    final byteMask = BigInt.from(0xFF);
+    var bdata = Uint8List(8);
+    for (int i = 0; i < 8; i++) {
+      bdata[i] = (number & byteMask).toInt();
+      number = number >> 8;
+    }
     output.addAll(bdata.buffer.asUint8List());
   }
 
diff --git a/serde-generate/runtime/dart/test/bincode_test.dart b/serde-generate/runtime/dart/test/bincode_test.dart
index 0ee29da2f..8f932afa7 100644
--- a/serde-generate/runtime/dart/test/bincode_test.dart
+++ b/serde-generate/runtime/dart/test/bincode_test.dart
@@ -95,6 +95,52 @@ void main() {
     expect(() => serializer.serializeInt32(-2147483649), throwsException);
   });
 
+  test('serializeInt64', () {
+    final serializer = BincodeSerializer();
+    serializer.serializeInt64(1);
+    expect(serializer.bytes,
+        Uint8List.fromList([1, 0, 0, 0, 0, 0, 0, 0]));
+    final deserializer = BincodeDeserializer(serializer.bytes);
+    expect(deserializer.deserializeInt64(), 1);
+  });
+
+  test('serializeInt64 negative', () {
+    final serializer = BincodeSerializer();
+    serializer.serializeInt64(-1);
+    expect(serializer.bytes,
+        Uint8List.fromList([0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF]));
+    final deserializer = BincodeDeserializer(serializer.bytes);
+    expect(deserializer.deserializeInt64(), -1);
+  });
+
+  test('serializeInt64 max safe integer', () {
+    // 2^53 - 1 (JS MAX_SAFE_INTEGER) — largest value portable across all platforms
+    final serializer = BincodeSerializer();
+    serializer.serializeInt64(9007199254740991);
+    expect(serializer.bytes,
+        Uint8List.fromList([0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x1F, 0x00]));
+    final deserializer = BincodeDeserializer(serializer.bytes);
+    expect(deserializer.deserializeInt64(), 9007199254740991);
+  });
+
+  test('serializeInt64 min safe integer', () {
+    // -(2^53 - 1) (JS MIN_SAFE_INTEGER)
+    final serializer = BincodeSerializer();
+    serializer.serializeInt64(-9007199254740991);
+    expect(serializer.bytes,
+        Uint8List.fromList([0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0xE0, 0xFF]));
+    final deserializer = BincodeDeserializer(serializer.bytes);
+    expect(deserializer.deserializeInt64(), -9007199254740991);
+  });
+
+  test('serializeInt64 typical timestamp', () {
+    // A realistic epoch-millis value (2026-03-23T00:00:00Z)
+    final serializer = BincodeSerializer();
+    serializer.serializeInt64(1774243200000);
+    final deserializer = BincodeDeserializer(serializer.bytes);
+    expect(deserializer.deserializeInt64(), 1774243200000);
+  });
+
   test('serializeString', () {
     final serializer = BincodeSerializer();
     serializer.serializeString('dummy text / ダミーテキスト');