diff --git a/types/hotwired__turbo/hotwired__turbo-tests.ts b/types/hotwired__turbo/hotwired__turbo-tests.ts
index 4bc74319abaabc..5e3f6a582d3f63 100644
--- a/types/hotwired__turbo/hotwired__turbo-tests.ts
+++ b/types/hotwired__turbo/hotwired__turbo-tests.ts
@@ -151,6 +151,13 @@ document.addEventListener("turbo:submit-end", function(event) {
     }
 });
 
+document.addEventListener("turbo:before-morph-attribute", function(event) {
+    // $ExpectType string
+    event.detail.attributeName;
+    // $ExpectType "update" | "remove"
+    event.detail.mutationType;
+});
+
 // Test start() function
 start();
 
diff --git a/types/hotwired__turbo/index.d.ts b/types/hotwired__turbo/index.d.ts
index bfd2d8a12449be..fa0c1ce206aec8 100644
--- a/types/hotwired__turbo/index.d.ts
+++ b/types/hotwired__turbo/index.d.ts
@@ -520,7 +520,7 @@ export type TurboMorphElementEvent = CustomEvent<{
 
 export type TurboBeforeMorphAttributeEvent = CustomEvent<{
     attributeName: string;
-    mutationType: "updated" | "removed";
+    mutationType: "update" | "remove";
 }>;
 
 export type TurboBeforeFrameMorphEvent = CustomEvent<{