diff --git a/.chronus/changes/decorator-common-types-ref-2025-0-16-11-22-58.md b/.chronus/changes/decorator-common-types-ref-2025-0-16-11-22-58.md
new file mode 100644
index 0000000000..bc843b7722
--- /dev/null
+++ b/.chronus/changes/decorator-common-types-ref-2025-0-16-11-22-58.md
@@ -0,0 +1,8 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-autorest"
+  - "@azure-tools/typespec-azure-resource-manager"
+---
+
+Add `@externalTypeRef` decorator, to been able to specify an external reference that should be used when emitting.
diff --git a/packages/typespec-autorest/src/openapi.ts b/packages/typespec-autorest/src/openapi.ts
index 3ef3f624bf..7be75d1a28 100644
--- a/packages/typespec-autorest/src/openapi.ts
+++ b/packages/typespec-autorest/src/openapi.ts
@@ -12,6 +12,7 @@ import {
 } from "@azure-tools/typespec-azure-core";
 import {
   getArmCommonTypeOpenAPIRef,
+  getExternalTypeRef,
   isArmCommonType,
   isAzureResource,
   isConditionallyFlattened,
@@ -936,6 +937,13 @@ export async function getOpenAPIForService(
       };
     }
 
+    const externalTypeRefUrl = getExternalTypeRef(program, type);
+    if (externalTypeRefUrl) {
+      return {
+        $ref: expandRef(externalTypeRefUrl),
+      };
+    }
+
     if (
       isArmCommonType(type) &&
       (type.kind === "Model" ||
diff --git a/packages/typespec-azure-resource-manager/README.md b/packages/typespec-azure-resource-manager/README.md
index 83f2165fc5..badb581b40 100644
--- a/packages/typespec-azure-resource-manager/README.md
+++ b/packages/typespec-azure-resource-manager/README.md
@@ -501,6 +501,7 @@ This allows sharing Azure Resource Manager resource types across specifications
 ### Azure.ResourceManager.Legacy
 
 - [`@customAzureResource`](#@customazureresource)
+- [`@externalTypeRef`](#@externaltyperef)
 
 #### `@customAzureResource`
 
@@ -518,3 +519,21 @@ but need to be identified as such.
 ##### Parameters
 
 None
+
+#### `@externalTypeRef`
+
+Specify an external reference that should be used when emitting this type.
+
+```typespec
+@Azure.ResourceManager.Legacy.externalTypeRef(jsonRef: valueof string)
+```
+
+##### Target
+
+`Model | ModelProperty`
+
+##### Parameters
+
+| Name    | Type             | Description                                                   |
+| ------- | ---------------- | ------------------------------------------------------------- |
+| jsonRef | `valueof string` | External reference(e.g. "../../common.json#/definitions/Foo") |
diff --git a/packages/typespec-azure-resource-manager/generated-defs/Azure.ResourceManager.ts b/packages/typespec-azure-resource-manager/generated-defs/Azure.ResourceManager.ts
index e956b75325..0955d90f24 100644
--- a/packages/typespec-azure-resource-manager/generated-defs/Azure.ResourceManager.ts
+++ b/packages/typespec-azure-resource-manager/generated-defs/Azure.ResourceManager.ts
@@ -4,6 +4,7 @@ import type {
   EnumValue,
   Interface,
   Model,
+  ModelProperty,
   Namespace,
   Operation,
   Type,
@@ -266,6 +267,17 @@ export type ResourceBaseTypeDecorator = (
   baseType: Type,
 ) => void;
 
+/**
+ *  * Specify an external reference that should be used when emitting this type.
+ *  *  @param jsonRef - External reference(e.g. "../../common.json#/definitions/Foo")
+ *
+ */
+export type ExternalTypeRefDecorator = (
+  context: DecoratorContext,
+  entity: Model | ModelProperty,
+  jsonRef: string,
+) => void;
+
 export type AzureResourceManagerDecorators = {
   armResourceCollectionAction: ArmResourceCollectionActionDecorator;
   armProviderNameValue: ArmProviderNameValueDecorator;
@@ -292,4 +304,5 @@ export type AzureResourceManagerDecorators = {
 
 export type AzureResourceManagerLegacyDecorators = {
   customAzureResource: CustomAzureResourceDecorator;
+  externalTypeRef: ExternalTypeRefDecorator;
 };
diff --git a/packages/typespec-azure-resource-manager/lib/Legacy/decorator.tsp b/packages/typespec-azure-resource-manager/lib/Legacy/decorator.tsp
index d00ce1ed28..4f483d67ec 100644
--- a/packages/typespec-azure-resource-manager/lib/Legacy/decorator.tsp
+++ b/packages/typespec-azure-resource-manager/lib/Legacy/decorator.tsp
@@ -7,3 +7,9 @@ namespace Azure.ResourceManager.Legacy;
  * but need to be identified as such.
  */
 extern dec customAzureResource(target: Model);
+
+/**
+ * Specify an external reference that should be used when emitting this type.
+ * @param jsonRef - External reference(e.g. "../../common.json#/definitions/Foo")
+ */
+extern dec externalTypeRef(entity: Model | ModelProperty, jsonRef: valueof string);
diff --git a/packages/typespec-azure-resource-manager/src/common-types.ts b/packages/typespec-azure-resource-manager/src/common-types.ts
index c91200dba9..13b5e2f984 100644
--- a/packages/typespec-azure-resource-manager/src/common-types.ts
+++ b/packages/typespec-azure-resource-manager/src/common-types.ts
@@ -14,7 +14,10 @@ import {
   isTypeSpecValueTypeOf,
 } from "@typespec/compiler";
 import { $useDependency, getVersion } from "@typespec/versioning";
-import { ArmCommonTypesVersionDecorator } from "../generated-defs/Azure.ResourceManager.js";
+import {
+  ArmCommonTypesVersionDecorator,
+  ExternalTypeRefDecorator,
+} from "../generated-defs/Azure.ResourceManager.js";
 import {
   ArmCommonTypeRecord,
   ArmCommonTypesDefaultVersion,
@@ -247,3 +250,15 @@ function resolveCommonTypesVersion(
     allVersions: allVersions ?? [],
   };
 }
+
+export const $externalTypeRef: ExternalTypeRefDecorator = (
+  context: DecoratorContext,
+  entity: Model | ModelProperty,
+  jsonRef: string,
+) => {
+  context.program.stateMap(ArmStateKeys.externalTypeRef).set(entity, jsonRef);
+};
+
+export function getExternalTypeRef(program: Program, entity: Type): string | undefined {
+  return program.stateMap(ArmStateKeys.externalTypeRef).get(entity);
+}
diff --git a/packages/typespec-azure-resource-manager/src/index.ts b/packages/typespec-azure-resource-manager/src/index.ts
index 7502dd992c..86865bc395 100644
--- a/packages/typespec-azure-resource-manager/src/index.ts
+++ b/packages/typespec-azure-resource-manager/src/index.ts
@@ -5,6 +5,7 @@ export {
   getArmCommonTypeOpenAPIRef,
   getArmCommonTypesVersion,
   getArmCommonTypesVersions,
+  getExternalTypeRef,
   isArmCommonType,
   type ArmCommonTypeVersions,
   type ArmCommonTypesResolutionOptions,
diff --git a/packages/typespec-azure-resource-manager/src/state.ts b/packages/typespec-azure-resource-manager/src/state.ts
index 7323a184d3..6ef6716671 100644
--- a/packages/typespec-azure-resource-manager/src/state.ts
+++ b/packages/typespec-azure-resource-manager/src/state.ts
@@ -14,6 +14,7 @@ export const ArmStateKeys = {
   armLibraryNamespaces: azureResourceManagerCreateStateSymbol("armLibraryNamespaces"),
   usesArmLibraryNamespaces: azureResourceManagerCreateStateSymbol("usesArmLibraryNamespaces"),
   armCommonTypesVersion: azureResourceManagerCreateStateSymbol("armCommonTypesVersion"),
+  externalTypeRef: azureResourceManagerCreateStateSymbol("externalTypeRef"),
 
   // resource.ts
   armResourcesCached: azureResourceManagerCreateStateSymbol("armResourcesCached"),
diff --git a/packages/typespec-azure-resource-manager/src/tsp-index.ts b/packages/typespec-azure-resource-manager/src/tsp-index.ts
index 2d0a27ea1b..ea04cf6ff9 100644
--- a/packages/typespec-azure-resource-manager/src/tsp-index.ts
+++ b/packages/typespec-azure-resource-manager/src/tsp-index.ts
@@ -3,7 +3,7 @@ import {
   AzureResourceManagerDecorators,
   AzureResourceManagerLegacyDecorators,
 } from "../generated-defs/Azure.ResourceManager.js";
-import { $armCommonTypesVersion } from "./common-types.js";
+import { $armCommonTypesVersion, $externalTypeRef } from "./common-types.js";
 import { $armLibraryNamespace, $armProviderNamespace, $useLibraryNamespace } from "./namespace.js";
 import {
   $armResourceAction,
@@ -57,6 +57,7 @@ export const $decorators = {
   } satisfies AzureResourceManagerDecorators,
   "Azure.ResourceManager.Legacy": {
     customAzureResource: $customAzureResource,
+    externalTypeRef: $externalTypeRef,
   } satisfies AzureResourceManagerLegacyDecorators,
 };
 
diff --git a/packages/typespec-azure-resource-manager/test/arm-common-types-registration.test.ts b/packages/typespec-azure-resource-manager/test/arm-common-types-registration.test.ts
index cd962b1c9c..1deb29bbac 100644
--- a/packages/typespec-azure-resource-manager/test/arm-common-types-registration.test.ts
+++ b/packages/typespec-azure-resource-manager/test/arm-common-types-registration.test.ts
@@ -1,8 +1,9 @@
 import type { Diagnostic, Model, ModelProperty, Namespace } from "@typespec/compiler";
 import { getService } from "@typespec/compiler";
 import { expectDiagnosticEmpty, expectDiagnostics } from "@typespec/compiler/testing";
+import { strictEqual } from "assert";
 import { describe, expect, it } from "vitest";
-import { findArmCommonTypeRecord } from "../src/common-types.js";
+import { findArmCommonTypeRecord, getExternalTypeRef } from "../src/common-types.js";
 import type { ArmCommonTypeRecord } from "../src/commontypes.private.decorators.js";
 import { createAzureResourceManagerTestRunner } from "./test-host.js";
 
@@ -205,3 +206,16 @@ describe("common parameters", () => {
     });
   });
 });
+
+describe("common types ref", () => {
+  it("set external reference", async () => {
+    const runner = await createAzureResourceManagerTestRunner();
+    const [{ Foo }, diagnostics] = await runner.compileAndDiagnose(`
+        @test @Azure.ResourceManager.Legacy.externalTypeRef("../common.json#/definitions/Foo")
+        model Foo {}
+      `);
+
+    expectDiagnosticEmpty(diagnostics);
+    strictEqual(getExternalTypeRef(runner.program, Foo), "../common.json#/definitions/Foo");
+  });
+});
diff --git a/website/src/content/docs/docs/libraries/azure-resource-manager/reference/decorators.md b/website/src/content/docs/docs/libraries/azure-resource-manager/reference/decorators.md
index 9b2cc18146..b5b1520c55 100644
--- a/website/src/content/docs/docs/libraries/azure-resource-manager/reference/decorators.md
+++ b/website/src/content/docs/docs/libraries/azure-resource-manager/reference/decorators.md
@@ -435,3 +435,21 @@ but need to be identified as such.
 #### Parameters
 
 None
+
+### `@externalTypeRef` {#@Azure.ResourceManager.Legacy.externalTypeRef}
+
+Specify an external reference that should be used when emitting this type.
+
+```typespec
+@Azure.ResourceManager.Legacy.externalTypeRef(jsonRef: valueof string)
+```
+
+#### Target
+
+`Model | ModelProperty`
+
+#### Parameters
+
+| Name    | Type             | Description                                                   |
+| ------- | ---------------- | ------------------------------------------------------------- |
+| jsonRef | `valueof string` | External reference(e.g. "../../common.json#/definitions/Foo") |
diff --git a/website/src/content/docs/docs/libraries/azure-resource-manager/reference/index.mdx b/website/src/content/docs/docs/libraries/azure-resource-manager/reference/index.mdx
index 88fbc6bd48..7c3821bb4d 100644
--- a/website/src/content/docs/docs/libraries/azure-resource-manager/reference/index.mdx
+++ b/website/src/content/docs/docs/libraries/azure-resource-manager/reference/index.mdx
@@ -269,6 +269,7 @@ npm install --save-peer @azure-tools/typespec-azure-resource-manager
 ### Decorators
 
 - [`@customAzureResource`](./decorators.md#@Azure.ResourceManager.Legacy.customAzureResource)
+- [`@externalTypeRef`](./decorators.md#@Azure.ResourceManager.Legacy.externalTypeRef)
 
 ### Models