diff --git a/.chronus/changes/flatten-2024-1-20-11-30-53.md b/.chronus/changes/flatten-2024-1-20-11-30-53.md
new file mode 100644
index 0000000000..01e62940e7
--- /dev/null
+++ b/.chronus/changes/flatten-2024-1-20-11-30-53.md
@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-autorest"
+---
+
+Support `@flattenProperty` decorator.
diff --git a/packages/typespec-autorest/src/openapi.ts b/packages/typespec-autorest/src/openapi.ts
index 7fe136815c..8ca25357ce 100644
--- a/packages/typespec-autorest/src/openapi.ts
+++ b/packages/typespec-autorest/src/openapi.ts
@@ -12,6 +12,7 @@ import {
   SdkContext,
   createSdkContext,
   getClientNameOverride,
+  shouldFlattenProperty,
 } from "@azure-tools/typespec-client-generator-core";
 import {
   ArrayModelType,
@@ -1945,6 +1946,13 @@ function createOAPIEmitter(
       }
     }
 
+    if (
+      typespecType.kind === "ModelProperty" &&
+      shouldFlattenProperty(tcgcSdkContext, typespecType)
+    ) {
+      newTarget["x-ms-client-flatten"] = true;
+    }
+
     attachExtensions(typespecType, newTarget);
 
     return typespecType.kind === "Scalar" || typespecType.kind === "ModelProperty"
diff --git a/packages/typespec-autorest/test/flatten.test.ts b/packages/typespec-autorest/test/flatten.test.ts
new file mode 100644
index 0000000000..e69b77b9bf
--- /dev/null
+++ b/packages/typespec-autorest/test/flatten.test.ts
@@ -0,0 +1,28 @@
+import { deepStrictEqual } from "assert";
+import { it } from "vitest";
+import { openApiFor } from "./test-host.js";
+
+it("applies x-ms-client-flatten for property marked with @flattenProperty", async () => {
+  const res = await openApiFor(
+    `
+    model Widget {
+      #suppress "deprecated" "for test"
+      @flattenProperty
+      properties?: WidgetProperties;
+    }
+
+    model WidgetProperties {
+    }
+    `
+  );
+  const model = res.definitions["Widget"]!;
+  deepStrictEqual(model, {
+    properties: {
+      properties: {
+        $ref: "#/definitions/WidgetProperties",
+        "x-ms-client-flatten": true,
+      },
+    },
+    type: "object",
+  });
+});