diff --git a/.chronus/changes/json-converter-2024-6-24-16-48-41.md b/.chronus/changes/json-converter-2024-6-24-16-48-41.md
new file mode 100644
index 0000000000..0fea7df5b8
--- /dev/null
+++ b/.chronus/changes/json-converter-2024-6-24-16-48-41.md
@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+Add `@hasJsonConverter` for csharp only to indicate if JSON converter is needed
diff --git a/docs/libraries/typespec-client-generator-core/reference/decorators.md b/docs/libraries/typespec-client-generator-core/reference/decorators.md
index fee0493ac5..374487dcdb 100644
--- a/docs/libraries/typespec-client-generator-core/reference/decorators.md
+++ b/docs/libraries/typespec-client-generator-core/reference/decorators.md
@@ -617,3 +617,30 @@ model Origin {
 @get
 op getModel(): Fish;
 ```
+
+### `@useSystemTextJsonConverter` {#@Azure.ClientGenerator.Core.useSystemTextJsonConverter}
+
+Whether a model needs the custom JSON converter, this is only used for backward compatibility for csharp.
+
+```typespec
+@Azure.ClientGenerator.Core.useSystemTextJsonConverter(scope?: valueof string)
+```
+
+#### Target
+
+`Model`
+
+#### Parameters
+
+| Name  | Type             | Description                                                                                                   |
+| ----- | ---------------- | ------------------------------------------------------------------------------------------------------------- |
+| scope | `valueof string` | The language scope you want this decorator to apply to. If not specified, will apply to all language emitters |
+
+#### Examples
+
+```typespec
+@useSystemTextJsonConverter
+model MyModel {
+  prop: string;
+}
+```
diff --git a/docs/libraries/typespec-client-generator-core/reference/index.mdx b/docs/libraries/typespec-client-generator-core/reference/index.mdx
index 49522d3657..d504e45c31 100644
--- a/docs/libraries/typespec-client-generator-core/reference/index.mdx
+++ b/docs/libraries/typespec-client-generator-core/reference/index.mdx
@@ -52,3 +52,4 @@ npm install --save-peer @azure-tools/typespec-client-generator-core
 - [`@override`](./decorators.md#@Azure.ClientGenerator.Core.override)
 - [`@protocolAPI`](./decorators.md#@Azure.ClientGenerator.Core.protocolAPI)
 - [`@usage`](./decorators.md#@Azure.ClientGenerator.Core.usage)
+- [`@useSystemTextJsonConverter`](./decorators.md#@Azure.ClientGenerator.Core.useSystemTextJsonConverter)
diff --git a/packages/typespec-client-generator-core/README.md b/packages/typespec-client-generator-core/README.md
index 9ccc9cdf08..479eb2cf9a 100644
--- a/packages/typespec-client-generator-core/README.md
+++ b/packages/typespec-client-generator-core/README.md
@@ -25,6 +25,7 @@ npm install @azure-tools/typespec-client-generator-core
 - [`@override`](#@override)
 - [`@protocolAPI`](#@protocolapi)
 - [`@usage`](#@usage)
+- [`@useSystemTextJsonConverter`](#@usesystemtextjsonconverter)
 
 #### `@access`
 
@@ -625,3 +626,30 @@ model Origin {
 @get
 op getModel(): Fish;
 ```
+
+#### `@useSystemTextJsonConverter`
+
+Whether a model needs the custom JSON converter, this is only used for backward compatibility for csharp.
+
+```typespec
+@Azure.ClientGenerator.Core.useSystemTextJsonConverter(scope?: valueof string)
+```
+
+##### Target
+
+`Model`
+
+##### Parameters
+
+| Name  | Type             | Description                                                                                                   |
+| ----- | ---------------- | ------------------------------------------------------------------------------------------------------------- |
+| scope | `valueof string` | The language scope you want this decorator to apply to. If not specified, will apply to all language emitters |
+
+##### Examples
+
+```typespec
+@useSystemTextJsonConverter
+model MyModel {
+  prop: string;
+}
+```
diff --git a/packages/typespec-client-generator-core/generated-defs/Azure.ClientGenerator.Core.ts b/packages/typespec-client-generator-core/generated-defs/Azure.ClientGenerator.Core.ts
index fdbb9f9126..7e83c78e80 100644
--- a/packages/typespec-client-generator-core/generated-defs/Azure.ClientGenerator.Core.ts
+++ b/packages/typespec-client-generator-core/generated-defs/Azure.ClientGenerator.Core.ts
@@ -483,3 +483,21 @@ export type OverrideDecorator = (
   override: Operation,
   scope?: string
 ) => void;
+
+/**
+ * Whether a model needs the custom JSON converter, this is only used for backward compatibility for csharp.
+ *
+ * @param scope The language scope you want this decorator to apply to. If not specified, will apply to all language emitters
+ * @example
+ * ```typespec
+ * @useSystemTextJsonConverter
+ * model MyModel {
+ *   prop: string;
+ * }
+ * ```
+ */
+export type UseSystemTextJsonConverterDecorator = (
+  context: DecoratorContext,
+  target: Model,
+  scope?: string
+) => void;
diff --git a/packages/typespec-client-generator-core/generated-defs/Azure.ClientGenerator.Core.ts-test.ts b/packages/typespec-client-generator-core/generated-defs/Azure.ClientGenerator.Core.ts-test.ts
index 1fe5974bec..08aefaa0d8 100644
--- a/packages/typespec-client-generator-core/generated-defs/Azure.ClientGenerator.Core.ts-test.ts
+++ b/packages/typespec-client-generator-core/generated-defs/Azure.ClientGenerator.Core.ts-test.ts
@@ -13,6 +13,7 @@ import {
   $override,
   $protocolAPI,
   $usage,
+  $useSystemTextJsonConverter,
 } from "@azure-tools/typespec-client-generator-core";
 import type {
   AccessDecorator,
@@ -28,6 +29,7 @@ import type {
   OverrideDecorator,
   ProtocolAPIDecorator,
   UsageDecorator,
+  UseSystemTextJsonConverterDecorator,
 } from "./Azure.ClientGenerator.Core.js";
 
 type Decorators = {
@@ -44,6 +46,7 @@ type Decorators = {
   $access: AccessDecorator;
   $flattenProperty: FlattenPropertyDecorator;
   $override: OverrideDecorator;
+  $useSystemTextJsonConverter: UseSystemTextJsonConverterDecorator;
 };
 
 /** An error here would mean that the exported decorator is not using the same signature. Make sure to have export const $decName: DecNameDecorator = (...) => ... */
@@ -61,4 +64,5 @@ const _: Decorators = {
   $access,
   $flattenProperty,
   $override,
+  $useSystemTextJsonConverter,
 };
diff --git a/packages/typespec-client-generator-core/lib/decorators.tsp b/packages/typespec-client-generator-core/lib/decorators.tsp
index e88fd4f8c0..46f1560311 100644
--- a/packages/typespec-client-generator-core/lib/decorators.tsp
+++ b/packages/typespec-client-generator-core/lib/decorators.tsp
@@ -467,3 +467,17 @@ extern dec flattenProperty(target: ModelProperty, scope?: valueof string);
  * ```
  */
 extern dec override(original: Operation, override: Operation, scope?: valueof string);
+
+/**
+ * Whether a model needs the custom JSON converter, this is only used for backward compatibility for csharp.
+ * @param scope The language scope you want this decorator to apply to. If not specified, will apply to all language emitters
+ *
+ * @example
+ * ```typespec
+ * @useSystemTextJsonConverter
+ * model MyModel {
+ *   prop: string;
+ * }
+ * ```
+ */
+extern dec useSystemTextJsonConverter(target: Model, scope?: valueof string);
diff --git a/packages/typespec-client-generator-core/src/decorators.ts b/packages/typespec-client-generator-core/src/decorators.ts
index 932fcda251..296c5a56cd 100644
--- a/packages/typespec-client-generator-core/src/decorators.ts
+++ b/packages/typespec-client-generator-core/src/decorators.ts
@@ -1135,3 +1135,9 @@ export function getOverriddenClientMethod(
 ): Operation | undefined {
   return getScopedDecoratorData(context, overrideKey, entity);
 }
+
+export const $useSystemTextJsonConverter: DecoratorFunction = (
+  context: DecoratorContext,
+  entity: Model,
+  scope?: LanguageScopes
+) => {};
diff --git a/packages/typespec-client-generator-core/test/types/general-decorators-list.test.ts b/packages/typespec-client-generator-core/test/types/general-decorators-list.test.ts
index 57ad1517e4..8608236957 100644
--- a/packages/typespec-client-generator-core/test/types/general-decorators-list.test.ts
+++ b/packages/typespec-client-generator-core/test/types/general-decorators-list.test.ts
@@ -327,4 +327,32 @@ describe("typespec-client-generator-core: general decorators list", () => {
       ]);
     });
   });
+
+  describe("csharp only decorator", () => {
+    it("@useSystemTextJsonConverter", async function () {
+      runner = await createSdkTestRunner(
+        {},
+        { additionalDecorators: ["Azure\\.ClientGenerator\\.Core\\.@useSystemTextJsonConverter"] }
+      );
+
+      await runner.compileWithBuiltInService(`
+        @useSystemTextJsonConverter("csharp")
+        model A {
+          id: string;
+        }
+
+        op test(): A;
+      `);
+
+      const models = runner.context.sdkPackage.models;
+      strictEqual(models.length, 1);
+      deepStrictEqual(models[0].decorators, [
+        {
+          name: "Azure.ClientGenerator.Core.@useSystemTextJsonConverter",
+          arguments: { scope: "csharp" },
+        },
+      ]);
+      expectDiagnostics(runner.context.diagnostics, []);
+    });
+  });
 });