diff --git a/packages/backend/src/core/entities/UserEntityService.ts b/packages/backend/src/core/entities/UserEntityService.ts
index a09369409b..9d73dc607d 100644
--- a/packages/backend/src/core/entities/UserEntityService.ts
+++ b/packages/backend/src/core/entities/UserEntityService.ts
@@ -409,7 +409,7 @@ export class UserEntityService implements OnModuleInit {
 				}),
 				pinnedPageId: profile!.pinnedPageId,
 				pinnedPage: profile!.pinnedPageId ? this.pageEntityService.pack(profile!.pinnedPageId, me) : null,
-				publicReactions: profile!.publicReactions,
+				publicReactions: this.isLocalUser(user) ? profile!.publicReactions : false, // https://github.com/misskey-dev/misskey/issues/12964
 				followersVisibility: profile!.followersVisibility,
 				followingVisibility: profile!.followingVisibility,
 				twoFactorEnabled: profile!.twoFactorEnabled,
diff --git a/packages/backend/src/server/api/endpoints/users/reactions.ts b/packages/backend/src/server/api/endpoints/users/reactions.ts
index 372ab80c4c..9404fb9447 100644
--- a/packages/backend/src/server/api/endpoints/users/reactions.ts
+++ b/packages/backend/src/server/api/endpoints/users/reactions.ts
@@ -9,6 +9,8 @@ import { Endpoint } from '@/server/api/endpoint-base.js';
 import { QueryService } from '@/core/QueryService.js';
 import { NoteReactionEntityService } from '@/core/entities/NoteReactionEntityService.js';
 import { DI } from '@/di-symbols.js';
+import { CacheService } from '@/core/CacheService.js';
+import { UserEntityService } from '@/core/entities/UserEntityService.js';
 import { ApiError } from '../../error.js';
 
 export const meta = {
@@ -34,6 +36,11 @@ export const meta = {
 			code: 'REACTIONS_NOT_PUBLIC',
 			id: '673a7dd2-6924-1093-e0c0-e68456ceae5c',
 		},
+		isRemoteUser: {
+			message: 'Currently unavailable to display reactions of remote users. See https://github.com/misskey-dev/misskey/issues/12964',
+			code: 'IS_REMOTE_USER',
+			id: '6b95fa98-8cf9-2350-e284-f0ffdb54a805',
+		},
 	},
 } as const;
 
@@ -59,12 +66,18 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 		@Inject(DI.noteReactionsRepository)
 		private noteReactionsRepository: NoteReactionsRepository,
 
+		private cacheService: CacheService,
+		private userEntityService: UserEntityService,
 		private noteReactionEntityService: NoteReactionEntityService,
 		private queryService: QueryService,
 	) {
 		super(meta, paramDef, async (ps, me) => {
-			const profile = await this.userProfilesRepository.findOneByOrFail({ userId: ps.userId });
+			const user = await this.cacheService.findUserById(ps.userId);
+			if (this.userEntityService.isRemoteUser(user)) {
+				throw new ApiError(meta.errors.isRemoteUser);
+			}
 
+			const profile = await this.userProfilesRepository.findOneByOrFail({ userId: ps.userId });
 			if ((me == null || me.id !== ps.userId) && !profile.publicReactions) {
 				throw new ApiError(meta.errors.reactionsNotPublic);
 			}