diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9e6db82..19c114f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -33,3 +33,4 @@
 - Implement `xsd:float` codec
 - Implement `xsd:unsignedInt` codec
 - Implement `xsd:integer` codec
+- Implement `xsd:negativeInteger` codec
diff --git a/README.md b/README.md
index e353563..9c017b4 100644
--- a/README.md
+++ b/README.md
@@ -36,7 +36,7 @@ This library aims to support the following XSD 1.1 built-in datatypes that are c
 |            `xsd:float` |      ✅      |        ✅        |    `double`    |     `dart:core`    |
 |          `xsd:integer` |      ✅      |        ✅        |    `BigInt`    |     `dart:core`    |
 | `xsd:nonPositiveInteger` |      ✅      |        ✅        |    `BigInt`    |     `dart:core`    |
-| `xsd:negativeInteger` |      ✅      |        ❌        |    `BigInt`    |     `dart:core`    |
+| `xsd:negativeInteger` |      ✅      |        ✅        |    `BigInt`    |     `dart:core`    |
 | `xsd:long` |      ✅      |        ✅        |    `BigInt`    |     `dart:core`    |
 | `xsd:int` |      ✅      |        ✅        |    `int`    |     `dart:core`    |
 | `xsd:short` |      ✅      |        ✅        |    `int`    |     `dart:core`    |
diff --git a/lib/src/codecs/codecs.dart b/lib/src/codecs/codecs.dart
index 4aac7f6..65d893f 100644
--- a/lib/src/codecs/codecs.dart
+++ b/lib/src/codecs/codecs.dart
@@ -30,3 +30,4 @@ export 'double/double_codec.dart';
 export 'float/float_codec.dart';
 export 'integer/integer_codec.dart';
 export 'unsigned_int/unsigned_int_codec.dart';
+export 'negative_integer/negative_integer_codec.dart';
diff --git a/lib/src/codecs/negative_integer/negative_integer_codec.dart b/lib/src/codecs/negative_integer/negative_integer_codec.dart
new file mode 100644
index 0000000..55cd4e7
--- /dev/null
+++ b/lib/src/codecs/negative_integer/negative_integer_codec.dart
@@ -0,0 +1,18 @@
+import 'dart:convert';
+
+import 'package:xsd/src/codecs/negative_integer/negative_integer_decoder.dart';
+import 'package:xsd/src/codecs/negative_integer/negative_integer_encoder.dart';
+
+/// A codec for xsd:negativeInteger.
+///
+/// Converts between a string representation of a negative integer and a
+/// [BigInt].
+class XsdNegativeIntegerCodec extends Codec<BigInt, String> {
+  const XsdNegativeIntegerCodec();
+
+  @override
+  Converter<String, BigInt> get decoder => const XsdNegativeIntegerDecoder();
+
+  @override
+  Converter<BigInt, String> get encoder => const XsdNegativeIntegerEncoder();
+}
diff --git a/lib/src/codecs/negative_integer/negative_integer_decoder.dart b/lib/src/codecs/negative_integer/negative_integer_decoder.dart
new file mode 100644
index 0000000..7b5186e
--- /dev/null
+++ b/lib/src/codecs/negative_integer/negative_integer_decoder.dart
@@ -0,0 +1,32 @@
+import 'dart:convert';
+
+import '../../helpers/whitespace.dart';
+
+class XsdNegativeIntegerDecoder extends Converter<String, BigInt> {
+  const XsdNegativeIntegerDecoder();
+
+  static final BigInt _maxValue = BigInt.from(-1);
+
+  @override
+  BigInt convert(String input) {
+    final str = processWhiteSpace(input, Whitespace.collapse);
+
+    if (str.isEmpty) {
+      throw const FormatException('The input string cannot be empty.');
+    }
+
+    final value = BigInt.tryParse(str);
+
+    if (value == null) {
+      throw FormatException('The input "$str" is not a valid integer.');
+    }
+
+    if (value > _maxValue) {
+      throw FormatException(
+        'The value "$value" must be less than or equal to $_maxValue.',
+      );
+    }
+
+    return value;
+  }
+}
diff --git a/lib/src/codecs/negative_integer/negative_integer_encoder.dart b/lib/src/codecs/negative_integer/negative_integer_encoder.dart
new file mode 100644
index 0000000..ba682d8
--- /dev/null
+++ b/lib/src/codecs/negative_integer/negative_integer_encoder.dart
@@ -0,0 +1,18 @@
+import 'dart:convert';
+
+class XsdNegativeIntegerEncoder extends Converter<BigInt, String> {
+  const XsdNegativeIntegerEncoder();
+
+  static final BigInt _maxValue = BigInt.from(-1);
+
+  @override
+  String convert(BigInt input) {
+    if (input > _maxValue) {
+      throw FormatException(
+        'The value "$input" must be less than or equal to $_maxValue.',
+      );
+    }
+
+    return input.toString();
+  }
+}
diff --git a/test/codecs/negative_integer_codec_test.dart b/test/codecs/negative_integer_codec_test.dart
new file mode 100644
index 0000000..ba9abd9
--- /dev/null
+++ b/test/codecs/negative_integer_codec_test.dart
@@ -0,0 +1,29 @@
+import 'package:test/test.dart';
+import 'package:xsd/xsd.dart';
+
+void main() {
+  group('XsdNegativeIntegerCodec', () {
+    const codec = XsdNegativeIntegerCodec();
+
+    test('should encode valid negative integer', () {
+      expect(codec.encode(BigInt.from(-1)), '-1');
+      expect(codec.encode(BigInt.from(-123)), '-123');
+    });
+
+    test('should decode valid negative integer', () {
+      expect(codec.decode('-1'), BigInt.from(-1));
+      expect(codec.decode('-123'), BigInt.from(-123));
+      expect(codec.decode('  -123  '), BigInt.from(-123));
+    });
+
+    test('should throw on invalid negative integer', () {
+      expect(() => codec.decode('0'), throwsFormatException);
+      expect(() => codec.decode('1'), throwsFormatException);
+      expect(() => codec.decode('abc'), throwsFormatException);
+      expect(() => codec.decode(''), throwsFormatException);
+      expect(() => codec.decode('   '), throwsFormatException);
+      expect(() => codec.encode(BigInt.zero), throwsFormatException);
+      expect(() => codec.encode(BigInt.one), throwsFormatException);
+    });
+  });
+}