diff --git a/.chronus/changes/tcgc-forbid-polymorphism-flatten-2024-6-23-22-12-38.md b/.chronus/changes/tcgc-forbid-polymorphism-flatten-2024-6-23-22-12-38.md
new file mode 100644
index 0000000000..0ec2696c11
--- /dev/null
+++ b/.chronus/changes/tcgc-forbid-polymorphism-flatten-2024-6-23-22-12-38.md
@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+Report error diagnostic when trying to flattening a model with polymorphism
\ No newline at end of file
diff --git a/packages/typespec-client-generator-core/src/decorators.ts b/packages/typespec-client-generator-core/src/decorators.ts
index d59ebeb36f..4b6f5b2bb1 100644
--- a/packages/typespec-client-generator-core/src/decorators.ts
+++ b/packages/typespec-client-generator-core/src/decorators.ts
@@ -17,6 +17,7 @@ import {
   Type,
   Union,
   createDiagnosticCollector,
+  getDiscriminator,
   getNamespaceFullName,
   getProjectedName,
   ignoreDiagnostics,
@@ -958,6 +959,13 @@ export function $flattenProperty(
   target: ModelProperty,
   scope?: LanguageScopes
 ) {
+  if (getDiscriminator(context.program, target.type)) {
+    reportDiagnostic(context.program, {
+      code: "flatten-polymorphism",
+      format: {},
+      target: target,
+    });
+  }
   setScopedDecoratorData(context, $flattenProperty, flattenPropertyKey, target, true, scope); // eslint-disable-line deprecation/deprecation
 }
 
diff --git a/packages/typespec-client-generator-core/src/lib.ts b/packages/typespec-client-generator-core/src/lib.ts
index 96409970e6..a951ea8544 100644
--- a/packages/typespec-client-generator-core/src/lib.ts
+++ b/packages/typespec-client-generator-core/src/lib.ts
@@ -186,6 +186,12 @@ export const $lib = createTypeSpecLibrary({
         default: paramMessage`Value in example file '${"relativePath"}' does not follow its definition:\n${"value"}`,
       },
     },
+    "flatten-polymorphism": {
+      severity: "error",
+      messages: {
+        default: `Cannot flatten property of polymorphic type.`,
+      },
+    },
   },
 });
 
diff --git a/packages/typespec-client-generator-core/test/decorators.test.ts b/packages/typespec-client-generator-core/test/decorators.test.ts
index 39066f3449..9e09587e18 100644
--- a/packages/typespec-client-generator-core/test/decorators.test.ts
+++ b/packages/typespec-client-generator-core/test/decorators.test.ts
@@ -2566,6 +2566,30 @@ describe("typespec-client-generator-core: decorators", () => {
         code: "decorator-wrong-target",
       });
     });
+
+    it("throws error when used on a polymorphism type", async () => {
+      const diagnostics = await runner.diagnose(`
+        @service
+        @test namespace MyService {
+          #suppress "deprecated" "@flattenProperty decorator is not recommended to use."
+          @test
+          model Model1{
+            @flattenProperty
+            child: Model2;
+          }
+
+          @test
+          @discriminator("kind")
+          model Model2{
+            kind: string;
+          }
+        }
+      `);
+
+      expectDiagnostics(diagnostics, {
+        code: "@azure-tools/typespec-client-generator-core/flatten-polymorphism",
+      });
+    });
   });
 
   describe("@clientName", () => {