diff --git a/packages/backend/src/mfm/from-html.ts b/packages/backend/src/mfm/from-html.ts
index 9b4346d2c..ffec44376 100644
--- a/packages/backend/src/mfm/from-html.ts
+++ b/packages/backend/src/mfm/from-html.ts
@@ -6,7 +6,7 @@ import { getSubjectHostFromUriAndUsernameCached } from "@/remote/resolve-user.js
 const urlRegex = /^https?:\/\/[\w\/:%#@$&?!()\[\]~.,=+\-]+/;
 const urlRegexFull = /^https?:\/\/[\w\/:%#@$&?!()\[\]~.,=+\-]+$/;
 
-export async function fromHtml(html: string, hashtagNames?: string[], basicMentionResolve: boolean = false): Promise<string> {
+export async function fromHtml(html: string, hashtagNames?: string[]): Promise<string> {
 	// some AP servers like Pixelfed use br tags as well as newlines
 	html = html.replace(/<br\s?\/?>\r?\n/gi, "\n");
 
@@ -73,9 +73,7 @@ export async function fromHtml(html: string, hashtagNames?: string[], basicMenti
 
 					if (part.length === 2 && href) {
 						//#region ホスト名部分が省略されているので復元する
-						const acct = basicMentionResolve
-							? `${txt}@${new URL(href.value).hostname}`
-							: `${txt}@${await getSubjectHostFromUriAndUsernameCached(href.value, txt)}`;
+						const acct = `${txt}@${await getSubjectHostFromUriAndUsernameCached(href.value, txt)}`;
 						text += acct;
 						//#endregion
 					} else if (part.length === 3) {
diff --git a/packages/backend/src/remote/resolve-user.ts b/packages/backend/src/remote/resolve-user.ts
index 2f0c6008e..b60106e28 100644
--- a/packages/backend/src/remote/resolve-user.ts
+++ b/packages/backend/src/remote/resolve-user.ts
@@ -190,9 +190,16 @@ export async function getSubjectHostFromUri(uri: string): Promise<string | null>
 }
 
 export async function getSubjectHostFromUriAndUsernameCached(uri: string, username: string): Promise<string | null> {
-	const hostname = new URL(uri).hostname;
+	const url = new URL(uri);
+	const hostname = url.hostname;
 	username = username.substring(1); // remove leading @ from username
 
+	// This resolves invalid mentions with the URL format https://host.tld/@user@otherhost.tld
+	const match = url.pathname.match(/^\/@(?<user>[a-zA-Z0-9_]+|$)@(?<host>[a-zA-Z0-9-.]+\.[a-zA-Z0-9-]+)$/)
+	if (match && match.groups?.host) {
+		return match.groups.host;
+	}
+
 	if (hostname === config.hostname) {
 		// user is local, return local account domain
 		return config.domain;
diff --git a/packages/backend/test/mfm.ts b/packages/backend/test/mfm.ts
index 5c9250048..5a42e420d 100644
--- a/packages/backend/test/mfm.ts
+++ b/packages/backend/test/mfm.ts
@@ -109,11 +109,7 @@ describe("fromHtml", () => {
 
 	it("mention", async () => {
 		assert.deepStrictEqual(
-			await fromHtml(
-				'<p>a <a href="https://joiniceshrimp.org/@user" class="u-url mention">@user</a> d</p>',
-				undefined,
-				false
-			),
+			await fromHtml('<p>a <a href="https://joiniceshrimp.org/@user" class="u-url mention">@user</a> d</p>'),
 			"a @user@joiniceshrimp.org d",
 		);
 	});