diff --git a/protobuf/lib/protobuf.dart b/protobuf/lib/protobuf.dart
index e2d5cdcb2..bed6b135e 100644
--- a/protobuf/lib/protobuf.dart
+++ b/protobuf/lib/protobuf.dart
@@ -13,6 +13,7 @@ import 'dart:typed_data' show TypedData, Uint8List, ByteData, Endian;
 import 'package:fixnum/fixnum.dart' show Int64;
 
 import 'src/protobuf/json_parsing_context.dart';
+import 'src/protobuf/json_serialization_context.dart';
 import 'src/protobuf/permissive_compare.dart';
 import 'src/protobuf/type_registry.dart';
 export 'src/protobuf/type_registry.dart' show TypeRegistry;
diff --git a/protobuf/lib/src/protobuf/generated_message.dart b/protobuf/lib/src/protobuf/generated_message.dart
index 1c7359774..504b07ea3 100644
--- a/protobuf/lib/src/protobuf/generated_message.dart
+++ b/protobuf/lib/src/protobuf/generated_message.dart
@@ -233,8 +233,9 @@ abstract class GeneratedMessage {
   /// message encoding a type not in [typeRegistry] is encountered, an
   /// error is thrown.
   Object? toProto3Json(
-          {TypeRegistry typeRegistry = const TypeRegistry.empty()}) =>
-      _writeToProto3Json(_fieldSet, typeRegistry);
+          {TypeRegistry typeRegistry = const TypeRegistry.empty(),
+          bool emitDefaults = false}) =>
+      _writeToProto3Json(_fieldSet, typeRegistry, emitDefaults);
 
   /// Merges field values from [json], a JSON object using proto3 encoding.
   ///
diff --git a/protobuf/lib/src/protobuf/json_serialization_context.dart b/protobuf/lib/src/protobuf/json_serialization_context.dart
new file mode 100644
index 000000000..1fdab29c5
--- /dev/null
+++ b/protobuf/lib/src/protobuf/json_serialization_context.dart
@@ -0,0 +1,18 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class JsonSerializationContext {
+  final bool emitDefaults;
+
+  JsonSerializationContext(this.emitDefaults);
+}
+
+class JsonSerializationException implements Exception {
+  final String message;
+
+  JsonSerializationException(this.message);
+
+  @override
+  String toString() => message;
+}
diff --git a/protobuf/lib/src/protobuf/proto3_json.dart b/protobuf/lib/src/protobuf/proto3_json.dart
index 6d35db920..0ab824e72 100644
--- a/protobuf/lib/src/protobuf/proto3_json.dart
+++ b/protobuf/lib/src/protobuf/proto3_json.dart
@@ -4,7 +4,10 @@
 
 part of protobuf;
 
-Object? _writeToProto3Json(_FieldSet fs, TypeRegistry typeRegistry) {
+Object? _writeToProto3Json(
+    _FieldSet fs, TypeRegistry typeRegistry, bool emitDefaults) {
+  var context = JsonSerializationContext(emitDefaults);
+
   String? convertToMapKey(dynamic key, int keyType) {
     var baseType = PbFieldType._baseType(keyType);
 
@@ -36,8 +39,8 @@ Object? _writeToProto3Json(_FieldSet fs, TypeRegistry typeRegistry) {
     if (fieldValue == null) return null;
 
     if (_isGroupOrMessage(fieldType!)) {
-      return _writeToProto3Json(
-          (fieldValue as GeneratedMessage)._fieldSet, typeRegistry);
+      return _writeToProto3Json((fieldValue as GeneratedMessage)._fieldSet,
+          typeRegistry, context.emitDefaults);
     } else if (_isEnum(fieldType)) {
       return (fieldValue as ProtobufEnum).name;
     } else {
@@ -88,25 +91,90 @@ Object? _writeToProto3Json(_FieldSet fs, TypeRegistry typeRegistry) {
 
   var result = <String, dynamic>{};
   for (var fieldInfo in fs._infosSortedByTag) {
-    var value = fs._values[fieldInfo.index!];
-    if (value == null || (value is List && value.isEmpty)) {
-      continue; // It's missing, repeated, or an empty byte array.
-    }
-    dynamic jsonValue;
-    if (fieldInfo.isMapField) {
-      jsonValue = (value as PbMap).map((key, entryValue) {
-        var mapEntryInfo = fieldInfo as MapFieldInfo;
-        return MapEntry(convertToMapKey(key, mapEntryInfo.keyFieldType!),
-            valueToProto3Json(entryValue, mapEntryInfo.valueFieldType));
-      });
-    } else if (fieldInfo.isRepeated) {
-      jsonValue = (value as PbListBase)
-          .map((element) => valueToProto3Json(element, fieldInfo.type))
-          .toList();
+    // if the value for this field is null, this function will return the
+    // default value, which simplifies the need to handle null for most cases
+    // below
+    var value = fs._getField(fieldInfo.tagNumber);
+
+    bool skipField = true;
+    Object? jsonValue;
+    if (fieldInfo.isRepeated) {
+      if (context.emitDefaults || (value as List).isNotEmpty) {
+        skipField = false;
+        jsonValue = (value as PbListBase)
+            .map((element) => valueToProto3Json(element, fieldInfo.type))
+            .toList();
+      }
+    } else if (fieldInfo.isMapField) {
+      if (context.emitDefaults || (value as Map).isNotEmpty) {
+        skipField = false;
+        jsonValue = (value as PbMap).map((key, entryValue) {
+          var mapEntryInfo = fieldInfo as MapFieldInfo;
+          return MapEntry(convertToMapKey(key, mapEntryInfo.keyFieldType!),
+              valueToProto3Json(entryValue, mapEntryInfo.valueFieldType));
+        });
+      }
+    } else if (_isBytes(fieldInfo.type)) {
+      if (context.emitDefaults || (value as List).isNotEmpty) {
+        skipField = false;
+        if ((value as List).isEmpty) {
+          jsonValue = null;
+        } else {
+          jsonValue = valueToProto3Json(value, fieldInfo.type);
+        }
+      }
+    } else if (fieldInfo.isEnum) {
+      // For enums, the default value is the first value listed in the enum's type definition
+      final defaultEnum = fieldInfo.enumValues!.first;
+      if (context.emitDefaults ||
+          (value as ProtobufEnum).name != defaultEnum.name) {
+        skipField = false;
+        jsonValue = valueToProto3Json(value, fieldInfo.type);
+      }
+    } else if (fieldInfo.isGroupOrMessage) {
+      final originalValue = fs._values[fieldInfo.index!];
+      if (context.emitDefaults || originalValue != null) {
+        skipField = false;
+        if (originalValue == null) {
+          jsonValue = null;
+        } else {
+          jsonValue = valueToProto3Json(value, fieldInfo.type);
+        }
+      }
+    } else if (PbFieldType._baseType(fieldInfo.type) ==
+        PbFieldType._STRING_BIT) {
+      if (context.emitDefaults || (value as String).isNotEmpty) {
+        skipField = false;
+        jsonValue = valueToProto3Json(value, fieldInfo.type);
+      }
+    } else if (PbFieldType._baseType(fieldInfo.type) == PbFieldType._BOOL_BIT) {
+      if (context.emitDefaults || value != false) {
+        skipField = false;
+        jsonValue = valueToProto3Json(value, fieldInfo.type);
+      }
+    } else if (value is Int64) {
+      if (context.emitDefaults || !value.isZero) {
+        skipField = false;
+        jsonValue = valueToProto3Json(value, fieldInfo.type);
+      }
+    } else if (value is int) {
+      if (context.emitDefaults || value != 0) {
+        skipField = false;
+        jsonValue = valueToProto3Json(value, fieldInfo.type);
+      }
+    } else if (value is double) {
+      if (context.emitDefaults || value != 0.0) {
+        skipField = false;
+        jsonValue = valueToProto3Json(value, fieldInfo.type);
+      }
     } else {
-      jsonValue = valueToProto3Json(value, fieldInfo.type);
+      throw JsonSerializationException(
+          'Unexpected field type for proto3 JSON serialization ${fieldInfo.type}');
+    }
+
+    if (!skipField) {
+      result[fieldInfo.name] = jsonValue;
     }
-    result[fieldInfo.name] = jsonValue;
   }
   // Extensions and unknown fields are not encoded by proto3 JSON.
   return result;
diff --git a/protobuf/test/json_test.dart b/protobuf/test/json_test.dart
index 6291ad8c7..093ab8b66 100644
--- a/protobuf/test/json_test.dart
+++ b/protobuf/test/json_test.dart
@@ -6,6 +6,7 @@ library json_test;
 import 'dart:convert';
 
 import 'package:fixnum/fixnum.dart' show Int64;
+import 'package:protobuf/protobuf.dart';
 import 'package:test/test.dart';
 
 import 'mock_util.dart' show T, mockEnumValues;
@@ -16,6 +17,32 @@ void main() {
     ..str = 'hello'
     ..int32s.addAll(<int>[1, 2, 3]);
 
+  final double doubleZeroVal = 0;
+  final exampleAllDefaults = T()
+    ..val = 0
+    ..str = ''
+    ..child = T()
+    ..int64 = Int64(0)
+    ..enm = ProtobufEnum(1, 'a')
+    ..dbl = doubleZeroVal
+    ..bl = false
+    ..byts = <int>[];
+  exampleAllDefaults.int32s.addAll(<int>[]);
+  exampleAllDefaults.mp.addAll(<String, String>{});
+
+  final double doubleSetVal = 1.34;
+  final exampleAllSet = T()
+    ..val = 32
+    ..str = 'the-string'
+    ..child = example
+    ..int64 = Int64(78)
+    ..enm = ProtobufEnum(1, 'b')
+    ..dbl = doubleSetVal
+    ..bl = true
+    ..byts = <int>[46, 28];
+  exampleAllSet.int32s.addAll(<int>[3, 4, 6]);
+  exampleAllSet.mp.addAll(<String, String>{'k1': 'v2'});
+
   test('testProto3JsonEnum', () {
     // No enum value specified.
     expect(example.hasEnm, isFalse);
@@ -115,6 +142,111 @@ void main() {
     final decoded = T()..mergeFromJsonMap(encoded);
     expect(decoded.int64, value);
   });
+
+  test('testToProto3Json', () {
+    var json = jsonEncode(example.toProto3Json());
+    checkProto3JsonMap(jsonDecode(json), 3);
+  });
+
+  test('testToProto3JsonNoFieldsSet', () {
+    final json = jsonEncode(T().toProto3Json());
+    expect(json.contains('{"val":42}'), isTrue);
+    final Map m = jsonDecode(json);
+    expect(m.length, 1);
+  });
+
+  test('testToProto3JsonFieldsSetToDefaults', () {
+    final json = jsonEncode(exampleAllDefaults.toProto3Json());
+    expect(json.contains('"child":{"val":42}'), isTrue);
+    final Map m = jsonDecode(json);
+    expect(m.isEmpty, isFalse);
+  });
+
+  test('testToProto3JsonFieldsSetToValues', () {
+    // verify the expected child is present
+    final json = jsonEncode(exampleAllSet.toProto3Json());
+    final expectedChild = '{"val":123,"str":"hello","int32s":[1,2,3]}';
+    expect(json.contains('"child":$expectedChild'), isTrue);
+    final jsonNoChild = json.replaceAll(expectedChild, '');
+
+    // verify the remaining parent object is accurate
+    checkExampleAllParentSetValues(jsonNoChild);
+  });
+
+  test('testToProto3JsonEmitDefaults', () {
+    final json = jsonEncode(example.toProto3Json(emitDefaults: true));
+    checkProto3JsonMap(jsonDecode(json), 10);
+  });
+
+  test('testToProto3JsonEmitDefaultsNoFieldsSet', () {
+    final json = jsonEncode(T().toProto3Json(emitDefaults: true));
+    expect(json.contains('"val":42,'), isTrue);
+    expect(json.contains('"str":"",'), isTrue);
+    expect(json.contains('"child":null,'), isTrue);
+    expect(json.contains('"int32s":[],'), isTrue);
+    expect(json.contains('"int64":"0",'), isTrue);
+    expect(json.contains('"enm":"a",'), isTrue);
+    expect(json.contains('"dbl":0.0,'), isTrue);
+    expect(json.contains('"bl":false,'), isTrue);
+    expect(json.contains('"byts":null'), isTrue);
+    expect(json.contains('"mp":{}'), isTrue);
+  });
+
+  test('testToProto3JsonEmitDefaultsFieldsSetToDefaults', () {
+    // verify the default child is present
+    final json =
+        jsonEncode(exampleAllDefaults.toProto3Json(emitDefaults: true));
+    final defaultChild =
+        '{"val":42,"str":"","child":null,"int32s":[],"int64":"0","enm":"a","dbl":0.0,"bl":false,"byts":null,"mp":{}}';
+    expect(json.contains('"child":$defaultChild'), isTrue);
+    final jsonNoChild = json.replaceAll(defaultChild, '');
+
+    // verify the remaining parent object is accurate
+    checkExampleAllParentSetDefaults(jsonNoChild);
+  });
+
+  test('testToProto3JsonEmitDefaultsFieldsSetToValues', () {
+    // verify the expected child is present
+    final json = jsonEncode(exampleAllSet.toProto3Json(emitDefaults: true));
+    final expectedChild =
+        '{"val":123,"str":"hello","child":null,"int32s":[1,2,3],"int64":"0","enm":"a","dbl":0.0,"bl":false,"byts":null,"mp":{}}';
+    expect(json.contains('"child":$expectedChild'), isTrue);
+    final jsonNoChild = json.replaceAll(expectedChild, '');
+
+    // verify the remaining parent object is accurate
+    checkExampleAllParentSetValues(jsonNoChild);
+  });
+}
+
+void checkExampleAllParentSetDefaults(String json) {
+  expect(json.contains('"val":0,'), isTrue);
+  expect(json.contains('"str":"",'), isTrue);
+  expect(json.contains('"int32s":[],'), isTrue);
+  expect(json.contains('"int64":"0",'), isTrue);
+  expect(json.contains('"enm":"a",'), isTrue);
+  expect(json.contains('"dbl":0.0,'), isTrue);
+  expect(json.contains('"bl":false,'), isTrue);
+  expect(json.contains('"byts":null'), isTrue);
+  expect(json.contains('"mp":{}'), isTrue);
+}
+
+void checkExampleAllParentSetValues(String json) {
+  expect(json.contains('"val":32,'), isTrue);
+  expect(json.contains('"str":"the-string",'), isTrue);
+  expect(json.contains('"int32s":[3,4,6],'), isTrue);
+  expect(json.contains('"int64":"78",'), isTrue);
+  expect(json.contains('"enm":"b",'), isTrue);
+  expect(json.contains('"dbl":1.34,'), isTrue);
+  expect(json.contains('"bl":true,'), isTrue);
+  expect(json.contains('"byts":"Lhw="'), isTrue);
+  expect(json.contains('"mp":{"k1":"v2"}'), isTrue);
+}
+
+void checkProto3JsonMap(Map m, int expectedLength) {
+  expect(m.length, expectedLength);
+  expect(m['val'], 123);
+  expect(m['str'], 'hello');
+  expect(m['int32s'], [1, 2, 3]);
 }
 
 void checkJsonMap(Map m) {
diff --git a/protobuf/test/map_mixin_test.dart b/protobuf/test/map_mixin_test.dart
index 2e40c5bcb..22a027d53 100644
--- a/protobuf/test/map_mixin_test.dart
+++ b/protobuf/test/map_mixin_test.dart
@@ -33,7 +33,18 @@ void main() {
 
     expect(r.isEmpty, false);
     expect(r.isNotEmpty, true);
-    expect(r.keys, ['val', 'str', 'child', 'int32s', 'int64', 'enm']);
+    expect(r.keys, [
+      'val',
+      'str',
+      'child',
+      'int32s',
+      'int64',
+      'enm',
+      'dbl',
+      'bl',
+      'byts',
+      'mp'
+    ]);
 
     expect(r['val'], 42);
     expect(r['str'], '');
@@ -42,7 +53,7 @@ void main() {
     expect(r['int32s'], []);
 
     var v = r.values;
-    expect(v.length, 6);
+    expect(v.length, 10);
     expect(v.first, 42);
     expect(v.toList()[1], '');
     expect(v.toList()[3].toString(), '[]');
diff --git a/protobuf/test/mock_util.dart b/protobuf/test/mock_util.dart
index f586f2d0d..b7cd92ca0 100644
--- a/protobuf/test/mock_util.dart
+++ b/protobuf/test/mock_util.dart
@@ -24,7 +24,12 @@ BuilderInfo mockInfo(String className, CreateBuilderFunc create) {
     ..e(7, 'enm', PbFieldType.OE,
         defaultOrMaker: mockEnumValues.first,
         valueOf: (i) => mockEnumValues.firstWhereOrNull((e) => e.value == i),
-        enumValues: mockEnumValues);
+        enumValues: mockEnumValues)
+    ..a(8, 'dbl', PbFieldType.OD)
+    ..aOB(9, 'bl')
+    ..a<int>(10, 'byts', PbFieldType.OY)
+    ..m<String, String>(11, 'mp',
+        keyFieldType: PbFieldType.OS, valueFieldType: PbFieldType.OS);
 }
 
 /// A minimal protobuf implementation for testing.
@@ -49,6 +54,18 @@ abstract class MockMessage extends GeneratedMessage {
 
   ProtobufEnum get enm => $_getN(5);
   bool get hasEnm => $_has(5);
+  set enm(x) => setField(7, x);
+
+  double get dbl => $_get(6, 0.0);
+  set dbl(x) => setField(8, x);
+
+  bool get bl => $_get(7, false);
+  set bl(x) => setField(9, x);
+
+  List<int> get byts => $_get(8, []);
+  set byts(x) => setField(10, x);
+
+  Map<String, String> get mp => $_getMap(9);
 
   @override
   GeneratedMessage clone() {
diff --git a/protoc_plugin/test/proto3_json_test.dart b/protoc_plugin/test/proto3_json_test.dart
index 02fb71ae2..6c2fb2d0a 100644
--- a/protoc_plugin/test/proto3_json_test.dart
+++ b/protoc_plugin/test/proto3_json_test.dart
@@ -107,14 +107,26 @@ final testAllTypesJson = {
   'defaultCord': '425'
 };
 
+// these fields have default values set for them within {@code setAllFields} and
+// thus will not be serialized to JSON adhering to the proto3 specification
+final keysWithDefaultValues = [
+  'defaultBool',
+  'defaultNestedEnum',
+  'defaultForeignEnum',
+  'defaultImportEnum'
+];
+
+final testAllTypesJsonNoDefaults = Map<String, Object>.from(testAllTypesJson)
+  ..removeWhere((k, v) => (keysWithDefaultValues.contains(k)));
+
 void main() {
   group('encode', () {
     test('testOutput', () {
-      expect(getAllSet().toProto3Json(), testAllTypesJson);
+      expect(getAllSet().toProto3Json(), testAllTypesJsonNoDefaults);
     });
 
     test('testFrozenOutput', () {
-      expect(getAllSet().freeze().toProto3Json(), testAllTypesJson);
+      expect(getAllSet().freeze().toProto3Json(), testAllTypesJsonNoDefaults);
     });
 
     test('testUnsignedOutput', () {
@@ -125,26 +137,51 @@ void main() {
 
       expect(message.toProto3Json(), {
         'optionalUint64': '17293822573397606400',
-        'optionalFixed64': '-1152921500311945215'
+        'optionalFixed64': '-1152921500311945215',
+        'defaultInt32': 41,
+        'defaultInt64': '42',
+        'defaultUint32': 43,
+        'defaultUint64': '44',
+        'defaultSint32': -45,
+        'defaultSint64': '46',
+        'defaultFixed32': 47,
+        'defaultFixed64': '48',
+        'defaultSfixed32': 49,
+        'defaultSfixed64': '-50',
+        'defaultFloat': 51.5,
+        'defaultDouble': 52000.0,
+        'defaultBool': true,
+        'defaultString': 'hello',
+        'defaultBytes': 'd29ybGQ=',
+        'defaultNestedEnum': 'BAR',
+        'defaultForeignEnum': 'FOREIGN_BAR',
+        'defaultImportEnum': 'IMPORT_BAR',
+        'defaultStringPiece': 'abc',
+        'defaultCord': '123'
       });
     });
 
     test('doubles', () {
-      void testValue(double value, Object expected) {
+      void testValue(double value, bool expectedInJson, Object expected) {
         var message = TestAllTypes()
           ..defaultFloat = value
           ..defaultDouble = value;
-        expect(
-            (message.toProto3Json() as Map)['defaultDouble'], equals(expected));
+
+        if (expectedInJson) {
+          expect((message.toProto3Json() as Map)['defaultDouble'],
+              equals(expected));
+        } else {
+          expect((message.toProto3Json() as Map)['defaultDouble'], isNull);
+        }
       }
 
-      testValue(-0.0, -0.0);
-      testValue(0.0, 0);
-      testValue(1.0, 1);
-      testValue(-1.0, -1);
-      testValue(double.nan, 'NaN');
-      testValue(double.infinity, 'Infinity');
-      testValue(double.negativeInfinity, '-Infinity');
+      testValue(-0.0, false, 0);
+      testValue(0.0, false, 0);
+      testValue(1.0, true, 1);
+      testValue(-1.0, true, -1);
+      testValue(double.nan, true, 'NaN');
+      testValue(double.infinity, true, 'Infinity');
+      testValue(double.negativeInfinity, true, '-Infinity');
     });
 
     test('map value', () {
@@ -255,7 +292,27 @@ void main() {
               .toProto3Json(typeRegistry: TypeRegistry([TestAllTypes()])),
           {
             '@type': 'type.googleapis.com/protobuf_unittest.TestAllTypes',
-            'optionalFixed64': '100'
+            'optionalFixed64': '100',
+            'defaultInt32': 41,
+            'defaultInt64': '42',
+            'defaultUint32': 43,
+            'defaultUint64': '44',
+            'defaultSint32': -45,
+            'defaultSint64': '46',
+            'defaultFixed32': 47,
+            'defaultFixed64': '48',
+            'defaultSfixed32': 49,
+            'defaultSfixed64': '-50',
+            'defaultFloat': 51.5,
+            'defaultDouble': 52000.0,
+            'defaultBool': true,
+            'defaultString': 'hello',
+            'defaultBytes': 'd29ybGQ=',
+            'defaultNestedEnum': 'BAR',
+            'defaultForeignEnum': 'FOREIGN_BAR',
+            'defaultImportEnum': 'IMPORT_BAR',
+            'defaultStringPiece': 'abc',
+            'defaultCord': '123',
           });
       expect(
           Any.pack(Timestamp.fromDateTime(DateTime.utc(1969, 7, 20, 20, 17)))