diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7a19fd4..13bca20 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,3 +21,4 @@
 - Implement `xsd:gMonth` codec and data type
 - Implement `xsd:unsignedShort` data type codec
 - Implement `xsd:positiveInteger` data type codec
+- Implement `xsd:nonPositiveInteger` data type codec
diff --git a/README.md b/README.md
index ef226d4..ebd2694 100644
--- a/README.md
+++ b/README.md
@@ -35,7 +35,7 @@ This library aims to support the following XSD 1.1 built-in datatypes that are c
 |           `xsd:double` |      ✅      |        ❌        |    `double`    |     `dart:core`    |
 |            `xsd:float` |      ✅      |        ❌        |    `double`    |     `dart:core`    |
 |          `xsd:integer` |      ✅      |        ❌        |    `BigInt`    |     `dart:core`    |
-| `xsd:nonPositiveInteger` |      ✅      |        ❌        |    `BigInt`    |     `dart:core`    |
+| `xsd:nonPositiveInteger` |      ✅      |        ✅        |    `BigInt`    |     `dart:core`    |
 | `xsd:negativeInteger` |      ✅      |        ❌        |    `BigInt`    |     `dart:core`    |
 | `xsd:long` |      ✅      |        ✅        |    `BigInt`    |     `dart:core`    |
 | `xsd:int` |      ✅      |        ✅        |    `int`    |     `dart:core`    |
diff --git a/lib/src/codecs/codecs.dart b/lib/src/codecs/codecs.dart
index cabc6ea..08b7f95 100644
--- a/lib/src/codecs/codecs.dart
+++ b/lib/src/codecs/codecs.dart
@@ -18,3 +18,4 @@ export 'g_year_month/year_month_codec.dart';
 export 'g_month/gregorian_month_codec.dart';
 export 'unsigned_short/unsigned_short_codec.dart';
 export 'positive_integer/positive_integer_codec.dart';
+export 'non_positive_integer/non_positive_integer_codec.dart';
diff --git a/lib/src/codecs/non_positive_integer/non_positive_integer_codec.dart b/lib/src/codecs/non_positive_integer/non_positive_integer_codec.dart
new file mode 100644
index 0000000..6440623
--- /dev/null
+++ b/lib/src/codecs/non_positive_integer/non_positive_integer_codec.dart
@@ -0,0 +1,18 @@
+import 'dart:convert';
+
+import 'package:xsd/src/codecs/non_positive_integer/non_positive_integer_decoder.dart';
+import 'package:xsd/src/codecs/non_positive_integer/non_positive_integer_encoder.dart';
+
+/// A codec for xsd:nonPositiveInteger.
+///
+/// Converts between a string representation of a non-positive integer and a
+/// [BigInt].
+class XsdNonPositiveIntegerCodec extends Codec<BigInt, String> {
+  const XsdNonPositiveIntegerCodec();
+
+  @override
+  Converter<String, BigInt> get decoder => const XsdNonPositiveIntegerDecoder();
+
+  @override
+  Converter<BigInt, String> get encoder => const XsdNonPositiveIntegerEncoder();
+}
diff --git a/lib/src/codecs/non_positive_integer/non_positive_integer_decoder.dart b/lib/src/codecs/non_positive_integer/non_positive_integer_decoder.dart
new file mode 100644
index 0000000..f028a8f
--- /dev/null
+++ b/lib/src/codecs/non_positive_integer/non_positive_integer_decoder.dart
@@ -0,0 +1,32 @@
+import 'dart:convert';
+
+import '../../helpers/whitespace.dart';
+
+class XsdNonPositiveIntegerDecoder extends Converter<String, BigInt> {
+  const XsdNonPositiveIntegerDecoder();
+
+  static final BigInt _maxValue = BigInt.zero;
+
+  @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 $_maxValue.',
+      );
+    }
+
+    return value;
+  }
+}
diff --git a/lib/src/codecs/non_positive_integer/non_positive_integer_encoder.dart b/lib/src/codecs/non_positive_integer/non_positive_integer_encoder.dart
new file mode 100644
index 0000000..47bb3fb
--- /dev/null
+++ b/lib/src/codecs/non_positive_integer/non_positive_integer_encoder.dart
@@ -0,0 +1,18 @@
+import 'dart:convert';
+
+class XsdNonPositiveIntegerEncoder extends Converter<BigInt, String> {
+  const XsdNonPositiveIntegerEncoder();
+
+  static final BigInt _maxValue = BigInt.zero;
+
+  @override
+  String convert(BigInt input) {
+    if (input > _maxValue) {
+      throw FormatException(
+        'The value "$input" must be less than or equal to $_maxValue.',
+      );
+    }
+
+    return input.toString();
+  }
+}
diff --git a/test/codecs/non_positive_integer_codec_test.dart b/test/codecs/non_positive_integer_codec_test.dart
new file mode 100644
index 0000000..004096c
--- /dev/null
+++ b/test/codecs/non_positive_integer_codec_test.dart
@@ -0,0 +1,56 @@
+import 'package:test/test.dart';
+import 'package:xsd/xsd.dart';
+
+void main() {
+  group('XsdNonPositiveIntegerCodec', () {
+    const codec = XsdNonPositiveIntegerCodec();
+
+    group('decoder', () {
+      test('should decode a valid non-positive integer string', () {
+        expect(codec.decode('0'), BigInt.zero);
+        expect(codec.decode('-1'), BigInt.from(-1));
+        expect(codec.decode('-123'), BigInt.from(-123));
+        expect(
+          codec.decode(
+            '-9999999999999999999999999999999999999999999999999999999999',
+          ),
+          BigInt.parse(
+            '-9999999999999999999999999999999999999999999999999999999999',
+          ),
+        );
+      });
+
+      test('should throw a FormatException for a positive integer', () {
+        expect(() => codec.decode('1'), throwsFormatException);
+      });
+
+      test('should throw a FormatException for an empty string', () {
+        expect(() => codec.decode(''), throwsFormatException);
+      });
+
+      test('should throw a FormatException for a non-integer string', () {
+        expect(() => codec.decode('abc'), throwsFormatException);
+      });
+    });
+
+    group('encoder', () {
+      test('should encode a valid non-positive integer', () {
+        expect(codec.encode(BigInt.zero), '0');
+        expect(codec.encode(BigInt.from(-1)), '-1');
+        expect(codec.encode(BigInt.from(-123)), '-123');
+        expect(
+          codec.encode(
+            BigInt.parse(
+              '-9999999999999999999999999999999999999999999999999999999999',
+            ),
+          ),
+          '-9999999999999999999999999999999999999999999999999999999999',
+        );
+      });
+
+      test('should throw a FormatException for a positive integer', () {
+        expect(() => codec.encode(BigInt.one), throwsFormatException);
+      });
+    });
+  });
+}