diff --git a/packages/freezed/CHANGELOG.md b/packages/freezed/CHANGELOG.md
index 776c541b..1d513723 100644
--- a/packages/freezed/CHANGELOG.md
+++ b/packages/freezed/CHANGELOG.md
@@ -1,3 +1,7 @@
+# 2.0.4
+
+Fixes a bug where using `@unfreezed` on unions somehow still generated immutable properties.
+
 # 2.0.3+1
 
 Update warning message when using abstract freezed classes
diff --git a/packages/freezed/lib/src/freezed_generator.dart b/packages/freezed/lib/src/freezed_generator.dart
index 79a4a897..b2ae1623 100644
--- a/packages/freezed/lib/src/freezed_generator.dart
+++ b/packages/freezed/lib/src/freezed_generator.dart
@@ -250,13 +250,26 @@ Read here: https://github.com/rrousselGit/freezed/blob/master/packages/freezed/C
     List<ConstructorDetails> constructorsNeedsGeneration,
   ) {
     return constructorsNeedsGeneration.first.parameters.allParameters
-        .where((parameter) {
-      return constructorsNeedsGeneration.every((constructor) {
-        return constructor.parameters.allParameters.any((p) {
-          return p.name == parameter.name && p.type == parameter.type;
-        });
-      });
-    }).toList();
+        .map((parameter) {
+          var hasAnyFinalProperty = false;
+          for (final constructor in constructorsNeedsGeneration) {
+            final matchingParameter =
+                constructor.parameters.allParameters.firstWhereOrNull((p) {
+              return p.name == parameter.name && p.type == parameter.type;
+            });
+
+            if (matchingParameter == null) return null;
+            if (matchingParameter.isFinal) hasAnyFinalProperty = true;
+          }
+
+          if (hasAnyFinalProperty) {
+            return parameter.copyWith(isFinal: true);
+          }
+
+          return parameter;
+        })
+        .whereNotNull()
+        .toList();
   }
 
   Future<List<ConstructorDetails>> _parseConstructorsNeedsGeneration(
diff --git a/packages/freezed/lib/src/templates/concrete_template.dart b/packages/freezed/lib/src/templates/concrete_template.dart
index 35904ad3..60121a3d 100644
--- a/packages/freezed/lib/src/templates/concrete_template.dart
+++ b/packages/freezed/lib/src/templates/concrete_template.dart
@@ -394,12 +394,14 @@ ${whenOrNullPrototype(data.constructors)} {
   }
 
   String get _abstractProperties {
-    return constructor.impliedProperties.map((p) {
-      if (commonProperties.any((element) => element.name == p.name)) {
-        return '@override ${p.unimplementedGetter}';
-      } else {
-        return '${p.unimplementedGetter}';
-      }
+    return constructor.impliedProperties.expand((p) {
+      return [
+        if (commonProperties.any((element) => element.name == p.name))
+          '@override ${p.abstractGetter}'
+        else
+          '${p.abstractGetter}',
+        if (!p.isFinal) p.abstractSetter,
+      ];
     }).join();
   }
 
diff --git a/packages/freezed/lib/src/templates/properties.dart b/packages/freezed/lib/src/templates/properties.dart
index 91e7127e..1c0a11ea 100644
--- a/packages/freezed/lib/src/templates/properties.dart
+++ b/packages/freezed/lib/src/templates/properties.dart
@@ -90,6 +90,14 @@ class Property {
         body: ' => throw $privConstUsedErrorVarName;',
       );
 
+  Getter get abstractGetter => Getter(
+        name: name,
+        type: type,
+        decorators: decorators,
+        doc: doc,
+        body: ';',
+      );
+
   Getter asGetter(String body) => Getter(
         name: name,
         type: type,
@@ -98,11 +106,20 @@ class Property {
         body: body,
       );
 
-  UnimplementedSetter get unimplementedSetter => UnimplementedSetter(
+  Setter get unimplementedSetter => Setter(
         name: name,
         type: type,
         decorators: decorators,
         doc: doc,
+        body: ' => throw $privConstUsedErrorVarName;',
+      );
+
+  Setter get abstractSetter => Setter(
+        name: name,
+        type: type,
+        decorators: decorators,
+        doc: doc,
+        body: ';',
       );
 
   Property copyWith({
@@ -158,23 +175,24 @@ class Getter {
   }
 }
 
-class UnimplementedSetter {
-  UnimplementedSetter({
+class Setter {
+  Setter({
     required String? type,
     required this.name,
     required this.decorators,
     required this.doc,
+    required this.body,
   }) : type = type ?? 'dynamic';
 
   final String type;
   final String name;
   final List<String> decorators;
   final String doc;
+  final String body;
 
   @override
   String toString() {
-    return '$doc${decorators.join()} set $name($type value) => '
-        'throw $privConstUsedErrorVarName;';
+    return '$doc${decorators.join()} set $name($type value)$body';
   }
 }
 
diff --git a/packages/freezed/test/integration/multiple_constructors.dart b/packages/freezed/test/integration/multiple_constructors.dart
index 5dd2b3d1..3df65a5f 100644
--- a/packages/freezed/test/integration/multiple_constructors.dart
+++ b/packages/freezed/test/integration/multiple_constructors.dart
@@ -2,6 +2,27 @@ import 'package:freezed_annotation/freezed_annotation.dart';
 
 part 'multiple_constructors.freezed.dart';
 
+@unfreezed
+class UnfreezedImmutableUnion with _$UnfreezedImmutableUnion {
+  factory UnfreezedImmutableUnion(final String a) =
+      DirectUnfreezedImmutableUnion;
+  factory UnfreezedImmutableUnion.named(String a) =
+      DirectUnfreezedImmutableUnionNamed;
+}
+
+@unfreezed
+class UnfreezedImmutableUnion2 with _$UnfreezedImmutableUnion2 {
+  factory UnfreezedImmutableUnion2(String a) = DirectUnfreezedImmutableUnion2;
+  factory UnfreezedImmutableUnion2.named(final String a) =
+      DirectUnfreezedImmutableUnionNamed2;
+}
+
+@unfreezed
+class MutableUnion with _$MutableUnion {
+  factory MutableUnion(String a, int b) = MutableUnion0;
+  factory MutableUnion.named(String a, int c) = MutableUnion1;
+}
+
 @freezed
 class DefaultValueNamedConstructor with _$DefaultValueNamedConstructor {
   factory DefaultValueNamedConstructor.a([@Default(42) int value]) =
diff --git a/packages/freezed/test/multiple_constructors_test.dart b/packages/freezed/test/multiple_constructors_test.dart
index 718f9128..6119d7ae 100644
--- a/packages/freezed/test/multiple_constructors_test.dart
+++ b/packages/freezed/test/multiple_constructors_test.dart
@@ -151,6 +151,47 @@ void main() {
       expect(errorResult.errors, isEmpty);
     });
 
+    test('can mutate unfreezed unions', () {
+      final unionFirst = MutableUnion0('a', 42);
+      final unionSecond = MutableUnion1('a', 42);
+      final MutableUnion downcast = unionFirst;
+
+      unionFirst
+        ..a = 'b'
+        ..b = 21;
+
+      unionSecond
+        ..a = 'b'
+        ..c = 21;
+
+      downcast.a = 'b';
+    });
+
+    test(
+        'cannot mutate shared property if one of the union has an immutable variant',
+        () async {
+      DirectUnfreezedImmutableUnionNamed('a').a = '';
+      DirectUnfreezedImmutableUnion2('a').a = '';
+
+      await expectLater(compile(r'''
+import 'multiple_constructors.dart';
+
+void main() {
+  UnfreezedImmutableUnion('').a;
+  UnfreezedImmutableUnion2('').a;
+}
+'''), completes);
+
+      await expectLater(compile(r'''
+import 'multiple_constructors.dart';
+
+void main() {
+  UnfreezedImmutableUnion('').a = '';
+  UnfreezedImmutableUnion2('').a = '';
+}
+'''), throwsCompileError);
+    });
+
     test('when works on unnamed constructors', () {
       expect(RequiredParams(a: 'a').when((a) => 21, second: (_) => 42), 21);
       expect(