diff --git a/apps/api/src/auth/presence.ts b/apps/api/src/auth/presence.ts
index a08d2b1..e76ac48 100644
--- a/apps/api/src/auth/presence.ts
+++ b/apps/api/src/auth/presence.ts
@@ -41,6 +41,15 @@ export async function onlineUserIds() {
   return new Set(members.map(Number).filter(Number.isInteger))
 }
 
+/**
+ * 在线人数。前台榜单页要的就是这一个数 —— 不必像 onlineUserIds 那样把成员全拉回来，
+ * ZCOUNT 让 Redis 自己数（O(log N)）。这里不顺手清过期成员：清理是写操作，
+ * 而这个端点是匿名可访问的。
+ */
+export async function onlineCount() {
+  return redis.zcount(PRESENCE_KEY, Date.now() - ONLINE_WINDOW_MS, "+inf")
+}
+
 /** 登出、被禁用、被踢下线：立刻从在线名单里摘掉，别等窗口自然过期 */
 export async function clearOnline(userId: number) {
   await redis.zrem(PRESENCE_KEY, String(userId))
diff --git a/apps/api/src/routes/account.ts b/apps/api/src/routes/account.ts
index e1f7dea..45408f1 100644
--- a/apps/api/src/routes/account.ts
+++ b/apps/api/src/routes/account.ts
@@ -33,6 +33,7 @@ import {
 import { Hono } from "hono"
 
 import { hashPassword } from "../auth/password"
+import { onlineUserIds } from "../auth/presence"
 import { optionalAuth, requireAuth, type AppEnv } from "../auth/middleware"
 import { config } from "../config"
 import { db, schema } from "../db"
@@ -40,7 +41,7 @@ import { failure, success } from "../http"
 import { JudgeStatus } from "../judge/status"
 import { getBooleanOption } from "../services/options"
 import { getUserProfileById } from "../services/profile"
-import { objectValue, queryInteger, sampleUser } from "./helpers"
+import { isTeacherOrAbove, objectValue, queryInteger, sampleUser } from "./helpers"
 
 export const accountRoutes = new Hono<AppEnv>()
 
@@ -184,7 +185,8 @@ accountRoutes.get("/rankings/users", optionalAuth, async (c) => {
   // 端点延迟从「四个来回相加」变成「最慢的那个」。越界页一条不剩，直接不发 SQL。
   const pageLimit = Math.max(0, Math.min(limit, LEADERBOARD_SIZE - offset))
 
-  const [totalRow, rows, me] = await Promise.all([
+  // 谁在线只给老师看，学生那边整列都是 null（见 rankProfileSchema.isOnline）
+  const [totalRow, rows, me, online] = await Promise.all([
     db.select({ value: count() }).from(schema.userProfile)
       .innerJoin(schema.user, eq(schema.userProfile.userId, schema.user.id))
       .where(leaderboardWhere).then(([row]) => row),
@@ -194,10 +196,11 @@ accountRoutes.get("/rankings/users", optionalAuth, async (c) => {
       .where(leaderboardWhere).orderBy(...leaderboardOrder)
       .limit(pageLimit).offset(offset),
     myLeaderboardRank(c.get("user")?.id),
+    isTeacherOrAbove(c.get("user")) ? onlineUserIds() : null,
   ])
 
   return success(c, userRankSchema.parse({
-    results: rows.map(serializeRankRow),
+    results: rows.map((row) => serializeRankRow(row, online)),
     total: Math.min(totalRow?.value ?? 0, LEADERBOARD_SIZE),
     me,
   }))
@@ -206,13 +209,14 @@ accountRoutes.get("/rankings/users", optionalAuth, async (c) => {
 function serializeRankRow({ profile, user }: {
   profile: typeof schema.userProfile.$inferSelect
   user: typeof schema.user.$inferSelect
-}) {
+}, online: Set<number> | null = null) {
   return rankProfileSchema.parse({
     id: profile.id,
     user: sampleUser(user, profile.realName),
     acceptedNumber: profile.acceptedNumber,
     submissionNumber: profile.submissionNumber,
     mood: profile.mood,
+    isOnline: online ? online.has(user.id) : null,
   })
 }
 
diff --git a/apps/api/src/routes/site.ts b/apps/api/src/routes/site.ts
index 1c0040f..c68b1ad 100644
--- a/apps/api/src/routes/site.ts
+++ b/apps/api/src/routes/site.ts
@@ -1,8 +1,9 @@
-import { quoteSchema, websiteConfigSchema } from "@oj2/contract"
+import { onlineCountSchema, quoteSchema, websiteConfigSchema } from "@oj2/contract"
 import { asc, desc, eq } from "drizzle-orm"
 import { Hono } from "hono"
 import { resolve } from "node:path"
 
+import { onlineCount } from "../auth/presence"
 import { config } from "../config"
 import { db, schema } from "../db"
 import { failure, success } from "../http"
@@ -25,6 +26,14 @@ siteRoutes.get("/site", async (c) => {
   }))
 })
 
+/**
+ * 当前在线人数。匿名可读 —— 一个聚合数字不暴露任何人的身份，
+ * 而榜单页本身就允许匿名看。谁在线是另一回事，只在 /rankings/users 里对老师下发。
+ */
+siteRoutes.get("/site/online", async (c) => {
+  return success(c, onlineCountSchema.parse({ count: await onlineCount() }))
+})
+
 // 数据集读不到时的兜底（本机 dev 没挂 data/hitokoto 就会走这里）
 const fallbackQuotes = [
   { hitokoto: "程序首先是写给人读的，其次才是让机器执行。", from: "Structure and Interpretation of Computer Programs" },
diff --git a/apps/web/src/oj/api.ts b/apps/web/src/oj/api.ts
index 15b5957..0712ccd 100644
--- a/apps/web/src/oj/api.ts
+++ b/apps/web/src/oj/api.ts
@@ -54,6 +54,7 @@ import type {
   Submission,
   SubmissionListPayload,
   SubmitCodePayload,
+  OnlineCount,
   WebsiteConfig,
   Tutorial,
   TutorialProgress,
@@ -71,6 +72,11 @@ export function getWebsiteConfig() {
   return api.get<WebsiteConfig>("site")
 }
 
+/** 当前在线人数。只有聚合数字，「谁在线」在榜单接口里、且只对老师下发 */
+export function getOnlineCount() {
+  return api.get<OnlineCount>("site/online")
+}
+
 export async function getProblemList(
   offset = 0,
   limit = 10,
diff --git a/apps/web/src/oj/rank/list.vue b/apps/web/src/oj/rank/list.vue
index 659eecb..4abf977 100644
--- a/apps/web/src/oj/rank/list.vue
+++ b/apps/web/src/oj/rank/list.vue
@@ -11,6 +11,7 @@ import { NButton, NFlex } from "naive-ui"
 import {
   getActivityRank,
   getClassRank,
+  getOnlineCount,
   getRank,
   getUserClassRank,
   getClassPK,
@@ -53,6 +54,8 @@ const query = reactive({
 })
 const message = useMessage()
 const rankChart = ref<Rank[]>([])
+/** 全站在线人数。只是个聚合数字；「谁在线」是每行的 isOnline，服务端只对老师下发 */
+const onlineCount = ref(0)
 const activityChart = ref<Rank[]>([])
 const duration = ref("months:1")
 const classData = ref<ClassRank[]>([])
@@ -182,6 +185,14 @@ const columns: DataTableColumn<Rank>[] = [
     width: 240,
     render: (row) =>
       h("div", { style: "display:flex;align-items:center;gap:6px" }, [
+        // isOnline 是三态：null 表示服务端没给（学生视角），只有 true 才点亮
+        row.isOnline
+          ? h("span", {
+              title: "在线（5 分钟内有活动）",
+              style:
+                "width:8px;height:8px;border-radius:50%;background:#18a058;flex:none",
+            })
+          : null,
         h(
           NButton,
           {
@@ -251,6 +262,11 @@ watch(
 )
 watch(duration, listActivity)
 
+async function listOnline() {
+  const res = await getOnlineCount()
+  onlineCount.value = res.count
+}
+
 async function listActivity() {
   const current = Date.now()
   const start = formatISO(sub(current, subOptions.value))
@@ -262,6 +278,7 @@ async function listActivity() {
     acceptedNumber: d.count,
     submissionNumber: 0,
     mood: null,
+    isOnline: null,
   }))
 }
 
@@ -286,6 +303,7 @@ onMounted(() => {
   // 再单发一次一模一样的 /rankings/users 只会让这张图排在日活后面出来。
   // 图只在挂载时定一次，翻页/改每页条数不该动它。
   init().then((results) => (rankChart.value = results.slice(0, 10)))
+  listOnline()
   listActivity()
   listClassRank()
   listMyClassRank()
@@ -523,6 +541,11 @@ watch(
     </n-grid>
     <n-card>
       <template #header>全服 Top100</template>
+      <template #header-extra>
+        <n-tag v-if="onlineCount > 0" round :bordered="false" type="success">
+          当前在线 {{ onlineCount }} 人
+        </n-tag>
+      </template>
       <n-data-table
         :data="data"
         :columns="columns"
diff --git a/apps/web/src/utils/types.ts b/apps/web/src/utils/types.ts
index 21106e4..621373c 100644
--- a/apps/web/src/utils/types.ts
+++ b/apps/web/src/utils/types.ts
@@ -396,7 +396,7 @@ export type ContestRank = Omit<
   submissionInfo: { [key: string]: SubmissionInfo }
 }
 
-export type { WebsiteConfig } from "@oj2/contract"
+export type { WebsiteConfig, OnlineCount } from "@oj2/contract"
 
 export type {
   JudgeServer as Server,
diff --git a/packages/contract/src/account.ts b/packages/contract/src/account.ts
index 3bea0c6..2391c8e 100644
--- a/packages/contract/src/account.ts
+++ b/packages/contract/src/account.ts
@@ -27,6 +27,12 @@ export const rankProfileSchema = z.object({
   acceptedNumber: z.number().int(),
   submissionNumber: z.number().int(),
   mood: z.string().nullable(),
+  /**
+   * 在线与否。**null 表示「这个调用方不该知道」** —— 学生之间互相盯着谁在刷题
+   * 不合适，所以只对老师及以上下发 true/false，其余一律 null。
+   * 三态是有意的：写成 boolean 的话，学生看到的 false 和真的离线分不开。
+   */
+  isOnline: z.boolean().nullable().default(null),
 })
 
 /**
diff --git a/packages/contract/src/site.ts b/packages/contract/src/site.ts
index ec03f02..e549793 100644
--- a/packages/contract/src/site.ts
+++ b/packages/contract/src/site.ts
@@ -11,6 +11,11 @@ export const websiteConfigSchema = z.object({
   enableMaxkb: z.boolean(),
 })
 
+/** 当前在线人数。只有聚合值 —— 「某某在不在线」是个人状态，不往匿名接口放 */
+export const onlineCountSchema = z.object({
+  count: z.number().int().nonnegative(),
+})
+
 export const quoteSchema = z.union([
   z.string(),
   z.record(z.string(), z.unknown()),
@@ -18,3 +23,4 @@ export const quoteSchema = z.union([
 
 export type WebsiteConfig = z.infer<typeof websiteConfigSchema>
 export type Quote = z.infer<typeof quoteSchema>
+export type OnlineCount = z.infer<typeof onlineCountSchema>