diff --git a/packages/backend/src/mfm/to-html.ts b/packages/backend/src/mfm/to-html.ts
index 0e876f8ae..7c941892f 100644
--- a/packages/backend/src/mfm/to-html.ts
+++ b/packages/backend/src/mfm/to-html.ts
@@ -119,7 +119,7 @@ export function toHtml(
 			const { username, host, acct } = node.props;
 			const remoteUserInfo = mentionedRemoteUsers.find(
 				(remoteUser) =>
-					remoteUser.username === username && remoteUser.host === host,
+					remoteUser.username.toLowerCase() === username.toLowerCase() && remoteUser.host === host,
 			);
 			a.href = remoteUserInfo
 				? remoteUserInfo.url
diff --git a/packages/backend/src/server/api/mastodon/helpers/mfm.ts b/packages/backend/src/server/api/mastodon/helpers/mfm.ts
index 1ad879654..c2d20d1db 100644
--- a/packages/backend/src/server/api/mastodon/helpers/mfm.ts
+++ b/packages/backend/src/server/api/mastodon/helpers/mfm.ts
@@ -140,7 +140,7 @@ export class MfmHelpers {
                 const { username, host} = node.props;
                 const remoteUserInfo = mentionedRemoteUsers.find(
                     (remoteUser) =>
-                        remoteUser.username === username && remoteUser.host === host,
+                        remoteUser.username.toLowerCase() === username.toLowerCase() && remoteUser.host === host,
                 );
                 const localpart = `@${username}`;
                 const isLocal = host === config.domain || (host == null && objectHost == null);