diff --git a/src/handlers/adminHandlers.ts b/src/handlers/adminHandlers.ts
index 71ed915..901e910 100644
--- a/src/handlers/adminHandlers.ts
+++ b/src/handlers/adminHandlers.ts
@@ -12,7 +12,7 @@ import { Timestamp } from "firebase-admin/firestore";
 import type { OrderStatus, ProductDoc } from "../types/reward";
 import { transitionOrder, updateOrderShipment } from "../services/orderService";
 import { invalidateRewardCatalog } from "../services/rewardCatalogService";
-import { listAdminUsers, searchAdminUsers } from "../services/adminUserService";
+import { appointAdmin, listAdminUsers, searchAdminUsers } from "../services/adminUserService";
 import {
   toAdminProductDto, toLedgerEntryDto, toOrderDto, toWalletDto,
   type AdminProductDto, type LedgerPageDto, type OrderPageDto,
@@ -132,6 +132,11 @@ export const admin = onRequest(async (req, res) => {
       return;
     }
 
+    if (tail === "users/admin" && req.method === "POST") {
+      res.json(await appointAdmin(req.body?.uid, adminUid));
+      return;
+    }
+
     if (tail === "users/list" && req.method === "GET") {
       const cursor = typeof req.query.cursor === "string" ? req.query.cursor : undefined;
       res.json(await listAdminUsers(req.query.limit, cursor));
diff --git a/src/services/adminUserService.ts b/src/services/adminUserService.ts
index 7f48730..370898e 100644
--- a/src/services/adminUserService.ts
+++ b/src/services/adminUserService.ts
@@ -14,6 +14,56 @@ import {
 
 const MAX_SEARCH_RESULTS = 30;
 
+async function attachAdminStatus(users: AdminUserDto[]): Promise<AdminUserDto[]> {
+  if (users.length === 0) return users;
+  const records = await auth.getUsers(users.map(({ uid }) => ({ uid })));
+  const adminUids = new Set(
+    records.users
+      .filter((record) => record.customClaims?.admin === true)
+      .map((record) => record.uid)
+  );
+  return users.map((user) => ({ ...user, isAdmin: adminUids.has(user.uid) }));
+}
+
+export interface AdminRoleUpdateResult {
+  uid: string;
+  isAdmin: true;
+  alreadyAdmin: boolean;
+}
+
+/** 기존 custom claims를 보존하면서 활성 유저에게 관리자 권한을 부여한다. */
+export async function appointAdmin(
+  uidRaw: unknown,
+  actorUid: string
+): Promise<AdminRoleUpdateResult> {
+  if (
+    typeof uidRaw !== "string" ||
+    uidRaw.length < 1 ||
+    uidRaw.length > 128 ||
+    uidRaw.trim() !== uidRaw ||
+    uidRaw.includes("/")
+  ) {
+    throw new HttpError(400, "invalid uid", "INVALID_INPUT");
+  }
+  const user = await getUser(uidRaw);
+  if (!user) throw new HttpError(404, "user not found", "USER_NOT_FOUND");
+  if (user.active === false) {
+    throw new HttpError(409, "inactive user cannot be admin", "INACTIVE_USER");
+  }
+
+  const record = await auth.getUser(uidRaw).catch(() => {
+    throw new HttpError(404, "auth user not found", "USER_NOT_FOUND");
+  });
+  if (record.customClaims?.admin === true) {
+    return { uid: uidRaw, isAdmin: true, alreadyAdmin: true };
+  }
+  await auth.setCustomUserClaims(uidRaw, {
+    ...(record.customClaims ?? {}),
+    admin: true,
+  });
+  return { uid: uidRaw, isAdmin: true, alreadyAdmin: false };
+}
+
 /**
  * 어드민 유저 통합 검색. 한 입력으로 세 경로를 함께 조회한다:
  * - uid 정확 일치 (공백 없는 10자 이상 입력일 때만 문서 1건 조회)
@@ -51,7 +101,7 @@ export async function searchAdminUsers(
     add(uid, user);
   }
 
-  return [...results.values()].slice(0, limit);
+  return attachAdminStatus([...results.values()].slice(0, limit));
 }
 
 /** 가입일 내림차순 유저 목록 페이지. */
@@ -61,7 +111,7 @@ export async function listAdminUsers(
 ): Promise<AdminUserPageDto> {
   const page = await listUsersByCreatedAt(Number(limitRaw) || 20, cursor);
   return {
-    items: page.items.map(({ uid, user }) => toAdminUserDto(uid, user)),
+    items: await attachAdminStatus(page.items.map(({ uid, user }) => toAdminUserDto(uid, user))),
     cursor: page.cursor,
   };
 }
diff --git a/src/types/dto/adminUserDto.ts b/src/types/dto/adminUserDto.ts
index 247b5bf..542ba38 100644
--- a/src/types/dto/adminUserDto.ts
+++ b/src/types/dto/adminUserDto.ts
@@ -15,6 +15,8 @@ export interface AdminUserDto {
   favoriteTeamCode?: TeamCode;
   /** active 필드가 없는 구(백필 전) 문서는 활성으로 간주한다. */
   active: boolean;
+  /** Firebase Auth custom claim의 admin=true 여부. */
+  isAdmin: boolean;
   createdAt?: string;
   deactivatedAt?: string;
 }
@@ -25,13 +27,14 @@ export interface AdminUserPageDto {
 }
 
 /** User 문서를 어드민 요약 DTO로 변환한다. 날짜는 UTC ISO 문자열, 옵션 필드는 키 자체를 생략. */
-export function toAdminUserDto(uid: string, user: User): AdminUserDto {
+export function toAdminUserDto(uid: string, user: User, isAdmin = false): AdminUserDto {
   return {
     uid,
     displayName: user.displayName,
     email: user.email,
     provider: user.provider,
     active: user.active !== false,
+    isAdmin,
     ...(user.photoUrl !== undefined ? { photoUrl: user.photoUrl } : {}),
     ...(user.favoriteTeamCode !== undefined ? { favoriteTeamCode: user.favoriteTeamCode } : {}),
     ...(user.createdAt ? { createdAt: toIsoOrUndefined(user.createdAt) } : {}),
diff --git a/tests/services/adminUserService.test.ts b/tests/services/adminUserService.test.ts
index ec5e033..7d4c123 100644
--- a/tests/services/adminUserService.test.ts
+++ b/tests/services/adminUserService.test.ts
@@ -1,7 +1,7 @@
 import { beforeAll, describe, expect, it } from "vitest";
 import { Timestamp } from "firebase-admin/firestore";
-import { firestore } from "../../src/firebase";
-import { listAdminUsers, searchAdminUsers } from "../../src/services/adminUserService";
+import { auth, firestore } from "../../src/firebase";
+import { appointAdmin, listAdminUsers, searchAdminUsers } from "../../src/services/adminUserService";
 
 // 다른 테스트가 만드는 users 문서와 섞이지 않도록 고유 접두어를 쓴다.
 const PREFIX = "검색전용";
@@ -30,6 +30,7 @@ const seedUsers = [
 beforeAll(async () => {
   for (const { uid, ...doc } of seedUsers) {
     await firestore.doc(`users/${uid}`).set(doc);
+    await auth.createUser({ uid, email: doc.email, displayName: doc.displayName });
   }
 });
 
@@ -62,6 +63,7 @@ describe("searchAdminUsers", () => {
     const results = await searchAdminUsers("admin-user-search-test-1", 10);
     expect(results[0]?.uid).toBe("admin-user-search-test-1");
     expect(results[0]?.displayName).toBe(`${PREFIX}호랑이`);
+    expect(results[0]?.isAdmin).toBe(false);
   });
 
   it("빈 q 는 400", async () => {
@@ -69,6 +71,36 @@ describe("searchAdminUsers", () => {
   });
 });
 
+describe("appointAdmin", () => {
+  it("기존 custom claims를 보존하면서 관리자 권한을 부여하고 재호출은 멱등이다", async () => {
+    const uid = "admin-user-search-test-1";
+    await auth.setCustomUserClaims(uid, { betaTester: true });
+
+    await expect(appointAdmin(uid, "granting-admin")).resolves.toEqual({
+      uid,
+      isAdmin: true,
+      alreadyAdmin: false,
+    });
+    const record = await auth.getUser(uid);
+    expect(record.customClaims).toMatchObject({
+      betaTester: true,
+      admin: true,
+    });
+    await expect(appointAdmin(uid, "granting-admin")).resolves.toMatchObject({
+      isAdmin: true,
+      alreadyAdmin: true,
+    });
+    const [dto] = await searchAdminUsers(uid, 10);
+    expect(dto.isAdmin).toBe(true);
+  });
+
+  it("비활성 유저는 관리자로 임명할 수 없다", async () => {
+    await expect(
+      appointAdmin("admin-user-search-test-2", "granting-admin")
+    ).rejects.toMatchObject({ status: 409, code: "INACTIVE_USER" });
+  });
+});
+
 describe("listAdminUsers", () => {
   it("가입일 내림차순 + 커서 페이지네이션", async () => {
     const first = await listAdminUsers(1);