diff --git a/packages/backend/assets/transparent.png b/packages/backend/assets/transparent.png
new file mode 100644
index 000000000..240ca4f8d
Binary files /dev/null and b/packages/backend/assets/transparent.png differ
diff --git a/packages/backend/src/core/CoreModule.ts b/packages/backend/src/core/CoreModule.ts
index eaa828091..ef29b39fd 100644
--- a/packages/backend/src/core/CoreModule.ts
+++ b/packages/backend/src/core/CoreModule.ts
@@ -140,6 +140,7 @@ import { ApQuestionService } from './activitypub/models/ApQuestionService.js';
 import { QueueModule } from './QueueModule.js';
 import { QueueService } from './QueueService.js';
 import { LoggerService } from './LoggerService.js';
+import { MastodonUserConverterService } from './mastodon/MastodonUserConverterService.js';
 import type { Provider } from '@nestjs/common';
 
 //#region 文字列ベースでのinjection用(循環参照対応のため)
@@ -422,6 +423,9 @@ const $ApQuestionService: Provider = { provide: 'ApQuestionService', useExisting
 		ApQuestionService,
 		QueueService,
 
+		// Mastodon
+		MastodonUserConverterService,
+
 		//#region 文字列ベースでのinjection用(循環参照対応のため)
 		$LoggerService,
 		$AccountMoveService,
@@ -696,6 +700,9 @@ const $ApQuestionService: Provider = { provide: 'ApQuestionService', useExisting
 		ApQuestionService,
 		QueueService,
 
+		// Mastodon
+		MastodonUserConverterService,
+
 		//#region 文字列ベースでのinjection用(循環参照対応のため)
 		$LoggerService,
 		$AccountMoveService,
diff --git a/packages/backend/src/core/mastodon/MastodonUserConverterService.ts b/packages/backend/src/core/mastodon/MastodonUserConverterService.ts
new file mode 100644
index 000000000..b5605e0e7
--- /dev/null
+++ b/packages/backend/src/core/mastodon/MastodonUserConverterService.ts
@@ -0,0 +1,208 @@
+import { Inject, Injectable } from '@nestjs/common';
+import * as mfm from 'mfm-js';
+import { MiLocalUser, MiUser } from '@/models/User.js';
+import { DI } from '@/di-symbols.js';
+import type { Config } from '@/config.js';
+import type { NotesRepository, UserProfilesRepository, UsersRepository } from '@/models/_.js';
+import { extractMentions } from '@/misc/extract-mentions.js';
+import { IMentionedRemoteUsers } from '@/models/Note.js';
+import { UserEntityService, UserRelation } from '../entities/UserEntityService.js';
+import { RoleService } from '../RoleService.js';
+import { MfmService } from '../MfmService.js';
+import { CustomEmojiService } from '../CustomEmojiService.js';
+import { ApPersonService } from '../activitypub/models/ApPersonService.js';
+import { IdService } from '../IdService.js';
+
+export type Account = {
+	id: string;
+	username: string;
+	acct: string;
+	url: string;
+	uri: string; // undocumented
+	display_name: string;
+	note: string;
+	avatar: string;
+	avatar_static: string;
+	header: string;
+	header_static: string;
+	locked: boolean;
+	fields: Field[];
+	emojis: Emoji[];
+	bot: boolean;
+	group: boolean;
+	discoverable: boolean | null;
+	noindex?: boolean | null;
+	moved?: Account | null;
+	suspended?: true;
+	limited?: true;
+	created_at: string;
+	last_status_at: string | null;
+	statuses_count: number;
+	followers_count: number;
+	following_count: number;
+};
+
+export type MutedAccount = Account | {
+	mute_expires_at: string | null;
+}
+
+export type SuggestedAccount = {
+	source: 'staff' | 'past_interactions' | 'global',
+	account: Account
+}
+
+export type Emoji = {
+	shortcode: string;
+	static_url: string;
+	url: string;
+	visible_in_picker: boolean;
+	category?: string;
+};
+
+export type Field = {
+	name: string;
+	value: string;
+	verified_at: string | null;
+};
+
+export type Source = {
+	privacy: string | null;
+	sensitive: boolean | null;
+	language: string | null;
+	note: string;
+	fields: Field[];
+};
+
+@Injectable()
+export class MastodonUserConverterService {
+	public constructor(
+		@Inject(DI.config)
+		private config: Config,
+
+		@Inject(DI.usersRepository)
+		private usersRepository: UsersRepository,
+
+		@Inject(DI.userProfilesRepository)
+		private userProfilesRepository: UserProfilesRepository,
+
+		@Inject(DI.notesRepository)
+		private notesRepository: NotesRepository,
+
+		private userEntityService: UserEntityService,
+		private roleService: RoleService,
+		private mfmService: MfmService,
+		private customEmojiService: CustomEmojiService,
+		private apPersonService: ApPersonService,
+		private idService: IdService,
+	) {}
+
+	public async encode(u: MiUser, me: MiLocalUser | null): Promise<Account | null> {
+		return await this.#encodeInner(u, me, true);
+	}
+
+	async #encodeInner(u: MiUser, me: MiLocalUser | null, recurse: boolean): Promise<Account | null> {
+		const profile = await this.userProfilesRepository.findOneByOrFail({ userId: u.id });
+		let acct = u.username;
+		let acctUrl = `https://${u.host ?? this.config.host}/@${u.username}`;
+		if (u.host) {
+			acct = `${u.username}@${u.host}`;
+			acctUrl = `https://${u.host}/@${u.username}`;
+		}
+		const meId = me ? me.id : null;
+		const isMe = meId === u.id;
+		const iAmModerator = me ? await this.roleService.isModerator(me) : false;
+		if (!iAmModerator && (u.isDeleted || u.isSuspended)) {
+			return null;
+		}
+		let relation: UserRelation | null = null;
+		if (meId && !isMe) {
+			relation = await this.userEntityService.getRelation(meId, u.id);
+		}
+		const followingCount =
+			(profile.followingVisibility === 'public') || isMe || iAmModerator ? u.followingCount :
+			(profile.followingVisibility === 'followers') && (relation && relation.isFollowing) ? u.followingCount :
+			0;
+		const followersCount =
+			(profile.followersVisibility === 'public') || isMe || iAmModerator ? u.followersCount :
+			(profile.followersVisibility === 'followers') && (relation && relation.isFollowing) ? u.followersCount :
+			0;
+		const avatarUrl = u.avatarUrl ?? this.userEntityService.getIdenticonUrl(u);
+		const bannerUrl = u.bannerUrl ?? `${this.config.url}/static-assets/transparent.png`;
+		const parsedDescription = mfm.parse(profile.description ?? '');
+		const parsedFields = profile.fields.map(({ name, value }) => ({ name, value, parsed: mfm.parse(value) }));
+		const mentions = await this.#extractMentions([...parsedDescription, ...parsedFields.flatMap(field => field.parsed)]);
+		const verifiedLinks = new Set(profile.verifiedLinks);
+		const fields = parsedFields.map(({ name, value, parsed }) => {
+			let verifiedAt: string | null = null;
+			if (verifiedLinks.has(value)) {
+				verifiedAt = new Date().toISOString();
+			}
+			return {
+				name,
+				value: this.mfmService.toHtml(parsed, mentions) ?? '',
+				verified_at: verifiedAt,
+			};
+		});
+		const emojis = Object.entries(await this.customEmojiService.populateEmojis(u.emojis, u.host)).map(([shortcode, url]) => ({
+			shortcode,
+			url,
+			static_url: url,
+			visible_in_picker: true,
+		}));
+		const lastStatus = await this.notesRepository.createQueryBuilder()
+			.where({ userId: u.id })
+			.orderBy('id', 'DESC')
+			.select('id')
+			.getOne();
+		const lastStatusAt = lastStatus ? this.idService.parse(lastStatus.id).date.toDateString() : null;
+		let moved = undefined;
+		if (recurse && u.movedToUri) {
+			moved = await this.#encodeInner(await this.apPersonService.resolvePerson(u.movedToUri), me, false);
+		}
+		return {
+			id: u.id,
+			username: u.username,
+			acct,
+			display_name: u.name ?? u.username,
+			locked: u.isLocked,
+			bot: u.isBot,
+			discoverable: u.isExplorable,
+			group: false, // unimplemented
+			created_at: u.createdAt.toISOString(),
+			note: this.mfmService.toHtml(parsedDescription, mentions) ?? '',
+			url: profile.url ?? acctUrl,
+			uri: u.uri ?? acctUrl,
+			avatar: avatarUrl,
+			avatar_static: avatarUrl,
+			header: bannerUrl,
+			header_static: bannerUrl,
+			followers_count: followersCount || 0,
+			following_count: followingCount || 0,
+			statuses_count: u.notesCount,
+			noindex: profile.noCrawle,
+			emojis,
+			last_status_at: lastStatusAt,
+			suspended: u.isSuspended || undefined,
+			fields,
+			moved,
+		};
+	}
+
+	async #extractMentions(nodes: mfm.MfmNode[]): Promise<IMentionedRemoteUsers> {
+		const mentions = extractMentions(nodes).filter(mention => mention.host !== null);
+		if (mentions.length === 0) {
+			return [];
+		}
+		let query = this.usersRepository.createQueryBuilder('user');
+		for (const { username, host } of mentions) {
+			query = query.orWhere(
+				'("user"."usernameLower" = :username AND host = :host)',
+				{ username: username.toLowerCase(), host: host?.toLowerCase() },
+			);
+		}
+		return await query
+			.innerJoinAndSelect(this.userProfilesRepository.metadata.targetName, 'profile', '"user".id = profile."userId"')
+			.select(['uri', 'url', 'username', 'host'])
+			.getRawMany();
+	}
+}
diff --git a/packages/backend/src/server/ServerModule.ts b/packages/backend/src/server/ServerModule.ts
index aafbf8060..42eab0093 100644
--- a/packages/backend/src/server/ServerModule.ts
+++ b/packages/backend/src/server/ServerModule.ts
@@ -46,10 +46,12 @@ import { UserListChannelService } from './api/stream/channels/user-list.js';
 import { RoleTimelineChannelService } from './api/stream/channels/role-timeline.js';
 import { ReversiChannelService } from './api/stream/channels/reversi.js';
 import { ReversiGameChannelService } from './api/stream/channels/reversi-game.js';
+import { MastodonEndpointsModule } from './api/MastodonEndpointsModule.js';
 
 @Module({
 	imports: [
 		EndpointsModule,
+		MastodonEndpointsModule,
 		CoreModule,
 	],
 	providers: [
diff --git a/packages/backend/src/server/api/ApiCallService.ts b/packages/backend/src/server/api/ApiCallService.ts
index cd15721b0..9c0421849 100644
--- a/packages/backend/src/server/api/ApiCallService.ts
+++ b/packages/backend/src/server/api/ApiCallService.ts
@@ -19,10 +19,11 @@ import { bindThis } from '@/decorators.js';
 import { RoleService } from '@/core/RoleService.js';
 import { IdentifiableError } from '@/misc/identifiable-error.js';
 import type { Config } from '@/config.js';
-import { ApiError } from './error.js';
+import { ApiError, MastodonApiError } from './error.js';
 import { RateLimiterService } from './RateLimiterService.js';
 import { ApiLoggerService } from './ApiLoggerService.js';
 import { AuthenticateService, AuthenticationError } from './AuthenticateService.js';
+import { IMastodonEndpoint } from './mastodon-endpoints.js';
 import type { FastifyRequest, FastifyReply } from 'fastify';
 import type { OnApplicationShutdown } from '@nestjs/common';
 import type { IEndpointMeta, IEndpoint } from './endpoints.js';
@@ -182,6 +183,32 @@ export class ApiCallService implements OnApplicationShutdown {
 		});
 	}
 
+	@bindThis
+	public async handleMastodonRequest(
+		endpoint: IMastodonEndpoint & { exec: any },
+		request: FastifyRequest<{ Body: Record<string, unknown> | undefined, Querystring: Record<string, unknown> }>,
+		reply: FastifyReply,
+	) {
+		const body = request.method === 'GET'
+			? request.query
+			: request.body;
+		const token = request.headers.authorization?.startsWith('Bearer ')
+			? request.headers.authorization.slice(7)
+			: null;
+		const [user] = await this.authenticateService.authenticate(token);
+		let res;
+		try {
+			res = await endpoint.exec(body, user, token, null, request.ip, request.headers);
+		} catch (e) {
+			if (e instanceof MastodonApiError) {
+				reply.status(e.httpStatusCode).send({ error: e.message });
+				return;
+			}
+			throw e;
+		}
+		reply.send(res);
+	}
+
 	@bindThis
 	private send(reply: FastifyReply, x?: any, y?: ApiError) {
 		if (x == null) {
diff --git a/packages/backend/src/server/api/ApiServerService.ts b/packages/backend/src/server/api/ApiServerService.ts
index 941d2f742..10e6272d6 100644
--- a/packages/backend/src/server/api/ApiServerService.ts
+++ b/packages/backend/src/server/api/ApiServerService.ts
@@ -17,6 +17,7 @@ import endpoints from './endpoints.js';
 import { ApiCallService } from './ApiCallService.js';
 import { SignupApiService } from './SignupApiService.js';
 import { SigninApiService } from './SigninApiService.js';
+import { IMastodonEndpoint, mastodonEndpoints } from './mastodon-endpoints.js';
 import type { FastifyInstance, FastifyPluginOptions } from 'fastify';
 
 @Injectable()
@@ -105,6 +106,32 @@ export class ApiServerService {
 			}
 		}
 
+		const createEndpoint = (endpoint: IMastodonEndpoint): IMastodonEndpoint & { exec: any } => ({
+			name: endpoint.name,
+			method: endpoint.method,
+			meta: endpoint.meta,
+			params: endpoint.params,
+			exec: this.moduleRef.get(`mep:${endpoint.method}:${endpoint.name}`, { strict: false }).exec,
+		});
+		const groupedMastodonEndpoints = Array.from(Map.groupBy(mastodonEndpoints.map(createEndpoint), endpoint => endpoint.name))
+			.map(([name, endpoints]) => ({ name, endpoints: new Map(endpoints.map(endpoint => [endpoint.method, endpoint])) }));
+		for (const { name, endpoints } of groupedMastodonEndpoints) {
+			fastify.all<{
+				Params: { endpoint: string; },
+				Body: Record<string, unknown>,
+				Querystring: Record<string, unknown>,
+			}>('/' + name, async (request, reply) => {
+				const ep = endpoints.get(request.method);
+				if (!ep) {
+					reply.code(405);
+					reply.send();
+					return;
+				}
+				await this.apiCallService.handleMastodonRequest(ep, request, reply);
+				return reply;
+			});
+		}
+
 		fastify.post<{
 			Body: {
 				username: string;
diff --git a/packages/backend/src/server/api/MastodonEndpointsModule.ts b/packages/backend/src/server/api/MastodonEndpointsModule.ts
new file mode 100644
index 000000000..099da28af
--- /dev/null
+++ b/packages/backend/src/server/api/MastodonEndpointsModule.ts
@@ -0,0 +1,19 @@
+import { Module, Provider } from '@nestjs/common';
+
+import { CoreModule } from '@/core/CoreModule.js';
+import * as mep___accounts_lookup_v1_get from './mastodon/accounts/lookup/v1/get.js';
+
+const $accounts_lookup_v1_get: Provider = { provide: 'mep:GET:v1/accounts/lookup', useClass: mep___accounts_lookup_v1_get.default };
+
+@Module({
+	imports: [
+		CoreModule,
+	],
+	providers: [
+		$accounts_lookup_v1_get,
+	],
+	exports: [
+		$accounts_lookup_v1_get,
+	],
+})
+export class MastodonEndpointsModule {}
diff --git a/packages/backend/src/server/api/ajv.ts b/packages/backend/src/server/api/ajv.ts
new file mode 100644
index 000000000..d6be6ec0a
--- /dev/null
+++ b/packages/backend/src/server/api/ajv.ts
@@ -0,0 +1,22 @@
+import _Ajv from 'ajv';
+
+const Ajv = _Ajv.default;
+
+const ajv = new Ajv({
+	useDefaults: true,
+});
+
+ajv.addFormat('misskey:id', /^[a-zA-Z0-9]+$/);
+ajv.addFormat('url', {
+	type: 'string',
+	validate: (url: string) => {
+		try {
+			new URL(url);
+			return true;
+		} catch (e) {
+			return false;
+		}
+	},
+});
+
+export { ajv };
diff --git a/packages/backend/src/server/api/endpoint-base.ts b/packages/backend/src/server/api/endpoint-base.ts
index 68f3d4c0f..54b3ac3f1 100644
--- a/packages/backend/src/server/api/endpoint-base.ts
+++ b/packages/backend/src/server/api/endpoint-base.ts
@@ -4,32 +4,13 @@
  */
 
 import * as fs from 'node:fs';
-import _Ajv from 'ajv';
 import type { Schema, SchemaType } from '@/misc/json-schema.js';
 import type { MiLocalUser } from '@/models/User.js';
 import type { MiAccessToken } from '@/models/AccessToken.js';
+import { ajv } from './ajv.js';
 import { ApiError } from './error.js';
 import type { IEndpointMeta } from './endpoints.js';
 
-const Ajv = _Ajv.default;
-
-const ajv = new Ajv({
-	useDefaults: true,
-});
-
-ajv.addFormat('misskey:id', /^[a-zA-Z0-9]+$/);
-ajv.addFormat('url', {
-	type: 'string',
-	validate: (url: string) => {
-		try {
-			new URL(url);
-			return true;
-		} catch (e) {
-			return false;
-		}
-	},
-});
-
 export type Response = Record<string, any> | void;
 
 type File = {
diff --git a/packages/backend/src/server/api/error.ts b/packages/backend/src/server/api/error.ts
index b05447050..fcdf1b939 100644
--- a/packages/backend/src/server/api/error.ts
+++ b/packages/backend/src/server/api/error.ts
@@ -23,3 +23,15 @@ export class ApiError extends Error {
 		this.info = info ?? undefined;
 	}
 }
+
+export class MastodonApiError extends Error {
+	constructor(message: string | undefined, public httpStatusCode: number) {
+		super(message ?? 'no error message available');
+	}
+}
+
+export class MastodonNotFoundError extends MastodonApiError {
+	constructor() {
+		super('Record not found', 404);
+	}
+}
diff --git a/packages/backend/src/server/api/mastodon-endpoint-base.ts b/packages/backend/src/server/api/mastodon-endpoint-base.ts
new file mode 100644
index 000000000..581485bcb
--- /dev/null
+++ b/packages/backend/src/server/api/mastodon-endpoint-base.ts
@@ -0,0 +1,39 @@
+import type { Schema, SchemaType } from '@/misc/json-schema.js';
+import { MiLocalUser } from '@/models/User.js';
+import { MiAccessToken } from '@/models/AccessToken.js';
+import { IMastodonEndpointMeta } from './mastodon-endpoints.js';
+import { ajv } from './ajv.js';
+import { MastodonApiError } from './error.js';
+
+type Executor<T extends IMastodonEndpointMeta, Ps extends Schema> =
+	(params: SchemaType<Ps>, user: T['requireCredential'] extends true ? MiLocalUser : MiLocalUser | null) => Promise<unknown>;
+
+export abstract class MastodonEndpoint<T extends IMastodonEndpointMeta, Ps extends Schema> {
+	public exec: (
+		params: SchemaType<Ps>,
+		user: T['requireCredential'] extends true ? MiLocalUser : MiLocalUser | null,
+		token: MiAccessToken | null,
+		file?: File,
+		ip?: string | null,
+		headers?: Record<string, string> | null,
+	) => Promise<unknown>;
+
+	constructor(meta: T, paramDef: Ps, exec: (
+		params: SchemaType<Ps>,
+		user: T['requireCredential'] extends true ? MiLocalUser : MiLocalUser | null,
+		token: MiAccessToken | null,
+		file?: File,
+		ip?: string | null,
+		headers?: Record<string, string> | null,
+	) => Promise<unknown>) {
+		const validate = ajv.compile(paramDef);
+		this.exec = (params, user, token, file, ip, headers) => {
+			const valid = validate(params);
+			if (!valid) {
+				const error = validate.errors![0];
+				throw new MastodonApiError(`${error.schemaPath}: ${error.message}`, 400);
+			}
+			return exec(params, user, token, file, ip, headers);
+		};
+	}
+}
diff --git a/packages/backend/src/server/api/mastodon-endpoints.ts b/packages/backend/src/server/api/mastodon-endpoints.ts
new file mode 100644
index 000000000..6c138c206
--- /dev/null
+++ b/packages/backend/src/server/api/mastodon-endpoints.ts
@@ -0,0 +1,30 @@
+import { Schema } from '@/misc/json-schema.js';
+import * as mep___accounts_lookup_v1_get from './mastodon/accounts/lookup/v1/get.js';
+
+const eps = [
+	['GET', 'v1/accounts/lookup', mep___accounts_lookup_v1_get],
+];
+
+export interface IMastodonEndpointMeta {
+	readonly requireCredential: boolean;
+}
+
+export interface IMastodonEndpoint {
+	name: string;
+	method: string;
+	meta: IMastodonEndpointMeta;
+	params: Schema;
+}
+
+export const mastodonEndpoints: IMastodonEndpoint[] = (eps as [string, string, any]).map(([method, name, ep]) => {
+	return {
+		name,
+		method,
+		get meta() {
+			return ep.meta;
+		},
+		get params() {
+			return ep.schema;
+		},
+	};
+});
diff --git a/packages/backend/src/server/api/mastodon/accounts/lookup/v1/get.ts b/packages/backend/src/server/api/mastodon/accounts/lookup/v1/get.ts
new file mode 100644
index 000000000..6cdc7aad3
--- /dev/null
+++ b/packages/backend/src/server/api/mastodon/accounts/lookup/v1/get.ts
@@ -0,0 +1,41 @@
+import { Injectable } from '@nestjs/common';
+import { MastodonEndpoint } from '@/server/api/mastodon-endpoint-base.js';
+import * as Acct from '@/misc/acct.js';
+import { RemoteUserResolveService } from '@/core/RemoteUserResolveService.js';
+import { MastodonUserConverterService } from '@/core/mastodon/MastodonUserConverterService.js';
+import { MastodonNotFoundError } from '@/server/api/error.js';
+
+export const meta = {
+	requireCredential: false,
+} as const;
+
+export const paramDef = {
+	type: 'object',
+	properties: {
+		acct: { type: 'string', minLength: 1 },
+	},
+	required: ['acct'],
+} as const;
+
+@Injectable()
+export default class extends MastodonEndpoint<typeof meta, typeof paramDef> { // eslint-disable-line import/no-default-export
+	constructor(
+		private remoteUserResolveService: RemoteUserResolveService,
+		private mastodonUserConverterService: MastodonUserConverterService,
+	) {
+		super(meta, paramDef, async (ps, me) => {
+			const { username, host } = Acct.parse(ps.acct);
+			let user;
+			try {
+				user = await this.remoteUserResolveService.resolveUser(username, host);
+			} catch (e) {
+				throw new MastodonNotFoundError();
+			}
+			const encoded = await this.mastodonUserConverterService.encode(user, me);
+			if (!encoded) {
+				throw new MastodonNotFoundError();
+			}
+			return encoded;
+		});
+	}
+}
diff --git a/packages/backend/test/e2e/endpoints.ts b/packages/backend/test/e2e/endpoints.ts
index e22f3b37a..2bde3dffb 100644
--- a/packages/backend/test/e2e/endpoints.ts
+++ b/packages/backend/test/e2e/endpoints.ts
@@ -10,7 +10,7 @@ import * as assert from 'assert';
 // https://github.com/node-fetch/node-fetch/pull/1664
 import { Blob } from 'node-fetch';
 import { MiUser } from '@/models/_.js';
-import { api, initTestDb, post, sendEnvUpdateRequest, signup, simpleGet, uploadFile } from '../utils.js';
+import { api, initTestDb, post, relativeFetch, sendEnvUpdateRequest, signup, simpleGet, uploadFile } from '../utils.js';
 import type * as misskey from 'misskey-js';
 
 describe('Endpoints', () => {
@@ -1062,4 +1062,16 @@ describe('Endpoints', () => {
 			assert.strictEqual(resCarol.body.memo, memoCarolToBob);
 		});
 	});
+
+	describe('v1/accounts/lookup', () => {
+		test('regular usage', async () => {
+			const response: any = await (await relativeFetch('api/v1/accounts/lookup?acct=alice')).json();
+			assert.strictEqual(response.username, 'alice');
+		});
+
+		test('404', async () => {
+			const response = await relativeFetch('api/v1/accounts/lookup?acct=fourohfour');
+			assert.strictEqual(response.status, 404);
+		});
+	});
 });