diff --git a/app/assets/javascripts/notes/components/comment_form.vue b/app/assets/javascripts/notes/components/comment_form.vue
index 1d6cb9485f7..b30d7fa9b73 100644
--- a/app/assets/javascripts/notes/components/comment_form.vue
+++ b/app/assets/javascripts/notes/components/comment_form.vue
@@ -115,8 +115,11 @@ export default {
     author() {
       return this.getUserData;
     },
-    canUpdateIssue() {
-      return this.getNoteableData.current_user.can_update;
+    canToggleIssueState() {
+      return (
+        this.getNoteableData.current_user.can_update &&
+        this.getNoteableData.state !== constants.MERGED
+      );
     },
     endpoint() {
       return this.getNoteableData.create_note_path;
@@ -415,7 +418,7 @@ append-right-10 comment-type-dropdown js-comment-type-dropdown droplab-dropdown"
               </div>
 
               <loading-button
-                v-if="canUpdateIssue"
+                v-if="canToggleIssueState"
                 :loading="isToggleStateButtonLoading"
                 :container-class="[
                   actionButtonClassNames,
diff --git a/app/assets/javascripts/notes/constants.js b/app/assets/javascripts/notes/constants.js
index fba3db8542c..bdfb6b8f105 100644
--- a/app/assets/javascripts/notes/constants.js
+++ b/app/assets/javascripts/notes/constants.js
@@ -7,6 +7,7 @@ export const COMMENT = 'comment';
 export const OPENED = 'opened';
 export const REOPENED = 'reopened';
 export const CLOSED = 'closed';
+export const MERGED = 'merged';
 export const EMOJI_THUMBSUP = 'thumbsup';
 export const EMOJI_THUMBSDOWN = 'thumbsdown';
 export const ISSUE_NOTEABLE_TYPE = 'issue';
diff --git a/changelogs/unreleased/do-not-reopen-merged-mr.yml b/changelogs/unreleased/do-not-reopen-merged-mr.yml
new file mode 100644
index 00000000000..14d1455cca4
--- /dev/null
+++ b/changelogs/unreleased/do-not-reopen-merged-mr.yml
@@ -0,0 +1,5 @@
+---
+title: Remove a "reopen merge request button" on a "merged" merge request
+merge_request: 26965
+author: Hiroyuki Sato
+type: fixed