diff --git a/.chronus/changes/unknown_format-2024-1-20-16-12-31.md b/.chronus/changes/unknown_format-2024-1-20-16-12-31.md
new file mode 100644
index 0000000000..5d405e1421
--- /dev/null
+++ b/.chronus/changes/unknown_format-2024-1-20-16-12-31.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+don't throw for unknown format
\ No newline at end of file
diff --git a/packages/typespec-client-generator-core/src/types.ts b/packages/typespec-client-generator-core/src/types.ts
index dbd0ab0b26..bec7ee3a06 100644
--- a/packages/typespec-client-generator-core/src/types.ts
+++ b/packages/typespec-client-generator-core/src/types.ts
@@ -134,7 +134,7 @@ function addFormatInfo(
         propertyType.kind = "azureLocation";
         break;
       default:
-        throw Error(`Unknown format ${format}`);
+        break;
     }
   }
 }
diff --git a/packages/typespec-client-generator-core/test/types.test.ts b/packages/typespec-client-generator-core/test/types.test.ts
index 79cdd23a46..c4bab9c57f 100644
--- a/packages/typespec-client-generator-core/test/types.test.ts
+++ b/packages/typespec-client-generator-core/test/types.test.ts
@@ -205,6 +205,22 @@ describe("typespec-client-generator-core: types", () => {
       }
     });
 
+    it("unknown format", async function () {
+      await runner.compileWithBuiltInService(
+        `
+        @usage(Usage.input | Usage.output)
+        @access(Access.public)
+        model Test {
+          @format("unknown")
+          unknownProp: string;
+        }
+      `
+      );
+      const models = getAllModelsAssertNoDiagnostics(runner.context);
+      strictEqual(models[0].kind, "model");
+      strictEqual(models[0].properties[0].type.kind, "string");
+    });
+
     it("known values", async function () {
       await runner.compileWithBuiltInService(
         `