diff --git a/pkg/json/CHANGELOG.md b/pkg/json/CHANGELOG.md
index b78404b21bc5..f7a36b99905e 100644
--- a/pkg/json/CHANGELOG.md
+++ b/pkg/json/CHANGELOG.md
@@ -1,3 +1,7 @@
+# 0.20.4
+
+- Allow custom `fromJson` to have an arbitrary parameter type.
+
 # 0.20.3
 
 - Add support for DateTime, serializing it to an ISO-8601 String.
diff --git a/pkg/json/lib/json.dart b/pkg/json/lib/json.dart
index 1a2f4ddfdabf..653acfee8c15 100644
--- a/pkg/json/lib/json.dart
+++ b/pkg/json/lib/json.dart
@@ -394,16 +394,16 @@ mixin _FromJson on _Shared {
     // Otherwise, check if `classDecl` has a `fromJson` constructor.
     final constructors = await builder.constructorsOf(classDecl);
     final fromJson = constructors
-        .firstWhereOrNull((c) => c.identifier.name == 'fromJson')
-        ?.identifier;
+        .firstWhereOrNull((c) => c.identifier.name == 'fromJson');
+
     if (fromJson != null) {
       return RawCode.fromParts([
         if (nullCheck != null) nullCheck,
-        fromJson,
+        fromJson.identifier,
         '(',
         jsonReference,
         ' as ',
-        introspectionData.jsonMapCode,
+        fromJson.positionalParameters.first.type.code,
         ')',
       ]);
     }
diff --git a/pkg/json/pubspec.yaml b/pkg/json/pubspec.yaml
index e2eb306c0d6b..5e06793d7534 100644
--- a/pkg/json/pubspec.yaml
+++ b/pkg/json/pubspec.yaml
@@ -5,7 +5,7 @@ description: >
   `toJson` encoding method.
 
 repository: https://github.com/dart-lang/sdk/tree/main/pkg/json
-version: 0.20.3
+version: 0.20.4
 environment:
   sdk: ^3.6.0-edge
 dependencies:
diff --git a/pkg/json/test/json_codable_test.dart b/pkg/json/test/json_codable_test.dart
index 15f48bc1ec4a..4c0daf9e3448 100644
--- a/pkg/json/test/json_codable_test.dart
+++ b/pkg/json/test/json_codable_test.dart
@@ -26,6 +26,11 @@ void main() {
         'mapOfSerializableField': {
           'c': {'x': 3}
         },
+        'customStringSerializationField': 'hi',
+        'customNullableStringSerialization': null,
+        'customMapSerializationField': {
+          'z': 'zzz',
+        }
       };
 
       var a = A.fromJson(json);
@@ -38,6 +43,11 @@ void main() {
       expect(a.listOfSerializableField.single.x, 1);
       expect(a.setOfSerializableField.single.x, 2);
       expect(a.mapOfSerializableField['c']!.x, 3);
+      expect(a.customStringSerializationField, CustomStringSerialization('hi'));
+      expect(a.customNullableStringSerialization,
+          equals(CustomNullableStringSerialization(null)));
+      expect(
+          a.customMapSerializationField, CustomMapSerialization({'z': 'zzz'}));
 
       expect(a.toJson(), equals(json));
     });
@@ -59,6 +69,8 @@ void main() {
         'nullableMapOfSerializableField': {
           'd': {'x': 3},
         },
+        'nullableCustomStringSerializationField': 'hi',
+        'nullableCustomMapSerializationField': {'z': 'zzz'},
       };
 
       var b = B.fromJson(json);
@@ -72,6 +84,10 @@ void main() {
       expect(b.nullableListOfSerializableField!.single.x, 1);
       expect(b.nullableSetOfSerializableField!.single.x, 2);
       expect(b.nullableMapOfSerializableField!['d']!.x, 3);
+      expect(b.nullableCustomStringSerializationField,
+          CustomStringSerialization('hi'));
+      expect(b.nullableCustomMapSerializationField,
+          CustomMapSerialization({'z': 'zzz'}));
 
       expect(b.toJson(), equals(json));
     });
@@ -87,6 +103,8 @@ void main() {
         'nullableListOfSerializableField': null,
         'nullableSetOfSerializableField': null,
         'nullableMapOfSerializableField': null,
+        'nullableCustomStringSerializationField': null,
+        'nullableCustomMapSerializationField': null,
       });
       expect(b.nullableBoolField, null);
       expect(b.nullableStringField, null);
@@ -97,6 +115,8 @@ void main() {
       expect(b.nullableListOfSerializableField, null);
       expect(b.nullableMapOfSerializableField, null);
       expect(b.nullableSetOfSerializableField, null);
+      expect(b.nullableCustomStringSerializationField, null);
+      expect(b.nullableCustomMapSerializationField, null);
 
       expect(b.toJson(), isEmpty);
     });
@@ -112,6 +132,8 @@ void main() {
       expect(b.nullableListOfSerializableField, null);
       expect(b.nullableMapOfSerializableField, null);
       expect(b.nullableSetOfSerializableField, null);
+      expect(b.nullableCustomStringSerializationField, null);
+      expect(b.nullableCustomMapSerializationField, null);
 
       expect(b.toJson(), isEmpty);
     });
@@ -140,6 +162,11 @@ void main() {
           null,
           {'a': 1, 'b': null},
         ],
+        'listOfCustomStringSerializables': [null, 'hi'],
+        'listOfCustomMapSerializables': [
+          null,
+          {'z': 'zzz'},
+        ],
         'setOfNullableDates': [
           null,
           '2024-11-12T03:42:29.108308',
@@ -159,6 +186,11 @@ void main() {
             'b': null,
           },
         ],
+        'setOfCustomStringSerializables': [null, 'hi'],
+        'setOfCustomMapSerializables': [
+          null,
+          {'z': 'zzz'}
+        ],
         'mapOfNullableDates': {
           'a': '2024-11-13T03:42:29.108308',
           'b': null,
@@ -175,6 +207,14 @@ void main() {
           'a': [null, 3],
           'b': null,
         },
+        'mapOfCustomStringSerializables': {
+          'a': null,
+          'b': 'hi',
+        },
+        'mapOfCustomMapSerializables': {
+          'a': null,
+          'b': {'z': 'zzz'},
+        },
       };
 
       var e = E.fromJson(json);
@@ -189,6 +229,13 @@ void main() {
             null,
             {'a': 1, 'b': null},
           ]));
+      expect(e.listOfCustomStringSerializables.first, null);
+      expect(e.listOfCustomStringSerializables[1],
+          CustomStringSerialization('hi'));
+      expect(e.listOfCustomMapSerializables.first, null);
+      expect(e.listOfCustomMapSerializables[1],
+          CustomMapSerialization({'z': 'zzz'}));
+
       expect(e.setOfNullableDates,
           equals([null, DateTime.parse('2024-11-12T03:42:29.108308')]));
       expect(e.setOfNullableInts, equals({null, 2}));
@@ -203,6 +250,13 @@ void main() {
               'b': null,
             },
           }));
+      expect(e.setOfCustomStringSerializables.first, null);
+      expect(e.setOfCustomStringSerializables.toList()[1],
+          CustomStringSerialization('hi'));
+      expect(e.setOfCustomMapSerializables.first, null);
+      expect(e.setOfCustomMapSerializables.toList()[1],
+          CustomMapSerialization({'z': 'zzz'}));
+
       expect(
           e.mapOfNullableDates,
           equals({
@@ -222,6 +276,12 @@ void main() {
         'a': {null, 3},
         'b': null,
       });
+      expect(e.mapOfCustomStringSerializables['a'], null);
+      expect(e.mapOfCustomStringSerializables['b'],
+          CustomStringSerialization('hi'));
+      expect(e.mapOfCustomMapSerializables['a'], null);
+      expect(e.mapOfCustomMapSerializables['b'],
+          CustomMapSerialization({'z': 'zzz'}));
 
       expect(e.toJson(), equals(json));
     });
@@ -257,6 +317,12 @@ class A {
   final Set<C> setOfSerializableField;
 
   final Map<String, C> mapOfSerializableField;
+
+  final CustomStringSerialization customStringSerializationField;
+
+  final CustomMapSerialization customMapSerializationField;
+
+  final CustomNullableStringSerialization customNullableStringSerialization;
 }
 
 @JsonCodable()
@@ -278,6 +344,10 @@ class B {
   final Set<C>? nullableSetOfSerializableField;
 
   final Map<String, C>? nullableMapOfSerializableField;
+
+  final CustomStringSerialization? nullableCustomStringSerializationField;
+
+  final CustomMapSerialization? nullableCustomMapSerializationField;
 }
 
 @JsonCodable()
@@ -300,6 +370,10 @@ class E {
 
   final List<Map<String, int?>?> listOfNullableMapsOfNullableInts;
 
+  final List<CustomStringSerialization?> listOfCustomStringSerializables;
+
+  final List<CustomMapSerialization?> listOfCustomMapSerializables;
+
   final Set<DateTime?> setOfNullableDates;
 
   final Set<int?> setOfNullableInts;
@@ -308,6 +382,10 @@ class E {
 
   final Set<Map<String, int?>?> setOfNullableMapsOfNullableInts;
 
+  final Set<CustomStringSerialization?> setOfCustomStringSerializables;
+
+  final Set<CustomMapSerialization?> setOfCustomMapSerializables;
+
   final Map<String, DateTime?> mapOfNullableDates;
 
   final Map<String, int?> mapOfNullableInts;
@@ -315,9 +393,69 @@ class E {
   final Map<String, C?> mapOfNullableSerializables;
 
   final Map<String, Set<int?>?> mapOfNullableSetsOfNullableInts;
+
+  final Map<String, CustomStringSerialization?> mapOfCustomStringSerializables;
+
+  final Map<String, CustomMapSerialization?> mapOfCustomMapSerializables;
 }
 
 @JsonCodable()
 class F {
   final int fieldWithDollarSign$;
 }
+
+class CustomStringSerialization {
+  final String a;
+
+  CustomStringSerialization(this.a);
+
+  String toJson() => a;
+
+  factory CustomStringSerialization.fromJson(String a) =>
+      CustomStringSerialization(a);
+
+  @override
+  bool operator ==(Object other) =>
+      other is CustomStringSerialization && a == other.a;
+
+  @override
+  int get hashCode => a.hashCode;
+}
+
+class CustomNullableStringSerialization {
+  final String? a;
+
+  CustomNullableStringSerialization(this.a);
+
+  String? toJson() => a;
+
+  factory CustomNullableStringSerialization.fromJson(String? a) =>
+      CustomNullableStringSerialization(a);
+
+  @override
+  bool operator ==(Object other) =>
+      other is CustomNullableStringSerialization && a == other.a;
+
+  @override
+  int get hashCode => a.hashCode;
+}
+
+class CustomMapSerialization {
+  final Map<String, dynamic> a;
+
+  CustomMapSerialization(this.a);
+
+  Map<String, dynamic> toJson() => a;
+
+  factory CustomMapSerialization.fromJson(Map<String, dynamic> a) =>
+      CustomMapSerialization(a);
+
+  @override
+  bool operator ==(Object other) {
+    return other is CustomMapSerialization &&
+        a.toString() == other.a.toString();
+  }
+
+  @override
+  int get hashCode => a.hashCode;
+}