diff --git a/src/components/admin/members-content.tsx b/src/components/admin/members-content.tsx
index c3ee801..e93a3c5 100644
--- a/src/components/admin/members-content.tsx
+++ b/src/components/admin/members-content.tsx
@@ -191,6 +191,20 @@ export function MembersContent() {
     },
   })
 
+  const bulkUpdateRoles = trpc.user.bulkUpdateRoles.useMutation({
+    onSuccess: (r) => {
+      const parts: string[] = []
+      if (r.updated > 0) parts.push(`Updated ${r.updated} user${r.updated === 1 ? '' : 's'}`)
+      if (r.alreadyHadRole > 0) parts.push(`${r.alreadyHadRole} already had role`)
+      toast.success(parts.join(' · ') || 'No changes')
+      setSelectedIds(new Set())
+      utils.user.list.invalidate()
+    },
+    onError: (error) => {
+      toast.error(error.message || 'Failed to update roles')
+    },
+  })
+
   const selectableUsers = useMemo(
     () => data?.users ?? [],
     [data?.users]
@@ -321,9 +335,29 @@ export function MembersContent() {
           <Card>
             <CardContent className="py-3 flex flex-wrap items-center justify-between gap-2">
               <p className="text-sm text-muted-foreground">
-                Selection persists across pages and filters.
+                {selectedIds.size > 0
+                  ? `${selectedIds.size} selected. Selection persists across pages and filters.`
+                  : 'Selection persists across pages and filters.'}
               </p>
               <div className="flex items-center gap-2">
+                {selectedIds.size > 0 && (
+                  <Button
+                    variant="outline"
+                    size="sm"
+                    onClick={() =>
+                      bulkUpdateRoles.mutate({
+                        userIds: Array.from(selectedIds),
+                        addRole: 'MENTOR',
+                      })
+                    }
+                    disabled={bulkUpdateRoles.isPending}
+                  >
+                    {bulkUpdateRoles.isPending ? (
+                      <Loader2 className="h-4 w-4 mr-2 animate-spin" />
+                    ) : null}
+                    Add MENTOR role
+                  </Button>
+                )}
                 <Button
                   variant="outline"
                   size="sm"
diff --git a/src/server/routers/user.ts b/src/server/routers/user.ts
index 7806e75..bae28a3 100644
--- a/src/server/routers/user.ts
+++ b/src/server/routers/user.ts
@@ -3,7 +3,7 @@ import { TRPCError } from '@trpc/server'
 import type { Prisma } from '@prisma/client'
 import { UserRole } from '@prisma/client'
 import { router, protectedProcedure, adminProcedure, superAdminProcedure, publicProcedure } from '../trpc'
-import { sendInvitationEmail, sendJuryInvitationEmail, sendMagicLinkEmail, sendPasswordResetEmail } from '@/lib/email'
+import { sendInvitationEmail, sendJuryInvitationEmail, sendMagicLinkEmail, sendMentorOnboardingEmail, sendPasswordResetEmail } from '@/lib/email'
 import { hashPassword, validatePassword } from '@/lib/password'
 import { attachAvatarUrls, getUserAvatarUrl } from '@/server/utils/avatar-url'
 import { logAudit } from '@/server/utils/audit'
@@ -1819,6 +1819,126 @@ export const userRouter = router({
       })
     }),
 
+  /**
+   * Bulk add/remove a single role across multiple users. Fires the mentor
+   * onboarding email exactly once per user when MENTOR is freshly added,
+   * idempotent via User.mentorOnboardingSentAt.
+   */
+  bulkUpdateRoles: adminProcedure
+    .input(
+      z.object({
+        userIds: z.array(z.string()).min(1).max(200),
+        addRole: z.nativeEnum(UserRole).optional(),
+        removeRole: z.nativeEnum(UserRole).optional(),
+      }).refine((d) => d.addRole || d.removeRole, {
+        message: 'Provide addRole or removeRole',
+      }),
+    )
+    .mutation(async ({ ctx, input }) => {
+      // Self-demote guard
+      if (input.removeRole === 'SUPER_ADMIN' && input.userIds.includes(ctx.user.id)) {
+        throw new TRPCError({
+          code: 'FORBIDDEN',
+          message: 'You cannot remove SUPER_ADMIN from self',
+        })
+      }
+      // Privilege guard: only SUPER_ADMIN can grant SUPER_ADMIN
+      if (input.addRole === 'SUPER_ADMIN' && ctx.user.role !== 'SUPER_ADMIN') {
+        throw new TRPCError({
+          code: 'FORBIDDEN',
+          message: 'Only super admins can grant super admin role',
+        })
+      }
+
+      const targets = await ctx.prisma.user.findMany({
+        where: { id: { in: input.userIds } },
+        select: { id: true, name: true, email: true, roles: true, mentorOnboardingSentAt: true },
+      })
+
+      let updated = 0
+      let alreadyHadRole = 0
+      const newlyMentor: typeof targets = []
+
+      const rolePriority: UserRole[] = [
+        'SUPER_ADMIN',
+        'PROGRAM_ADMIN',
+        'JURY_MEMBER',
+        'MENTOR',
+        'OBSERVER',
+        'AWARD_MASTER',
+        'APPLICANT',
+        'AUDIENCE',
+      ]
+
+      for (const u of targets) {
+        const current = new Set(u.roles)
+        const next = new Set(current)
+
+        if (input.addRole) {
+          if (current.has(input.addRole)) {
+            alreadyHadRole++
+            continue
+          }
+          next.add(input.addRole)
+        }
+        if (input.removeRole) {
+          if (!current.has(input.removeRole)) {
+            alreadyHadRole++
+            continue
+          }
+          next.delete(input.removeRole)
+        }
+        if (next.size === 0) next.add('APPLICANT' as UserRole) // safety: never empty
+
+        const nextArr = Array.from(next)
+        const primary = rolePriority.find((r) => nextArr.includes(r)) ?? nextArr[0]
+
+        const isFreshMentor =
+          input.addRole === 'MENTOR' && !current.has('MENTOR') && !u.mentorOnboardingSentAt
+
+        await ctx.prisma.user.update({
+          where: { id: u.id },
+          data: {
+            roles: nextArr,
+            role: primary,
+            ...(isFreshMentor ? { mentorOnboardingSentAt: new Date() } : {}),
+          },
+        })
+
+        await logAudit({
+          prisma: ctx.prisma,
+          userId: ctx.user.id,
+          action: input.addRole ? 'USER_ROLE_ADD' : 'USER_ROLE_REMOVE',
+          entityType: 'User',
+          entityId: u.id,
+          detailsJson: {
+            addRole: input.addRole,
+            removeRole: input.removeRole,
+            before: u.roles,
+            after: nextArr,
+          },
+          ipAddress: ctx.ip,
+          userAgent: ctx.userAgent,
+        })
+
+        if (isFreshMentor) newlyMentor.push(u)
+        updated++
+      }
+
+      // Send onboarding emails outside the per-user mutation loop. Errors
+      // caught and logged — we don't roll back the role grant if email
+      // delivery fails.
+      for (const u of newlyMentor) {
+        try {
+          await sendMentorOnboardingEmail(u.email, u.name)
+        } catch (err) {
+          console.error('[bulkUpdateRoles] mentor-onboarding email failed for', u.email, err)
+        }
+      }
+
+      return { updated, alreadyHadRole, total: targets.length }
+    }),
+
   /**
    * List applicant users with project info for admin bulk-invite page.
    */
diff --git a/tests/unit/bulk-role-updates.test.ts b/tests/unit/bulk-role-updates.test.ts
new file mode 100644
index 0000000..5cb763c
--- /dev/null
+++ b/tests/unit/bulk-role-updates.test.ts
@@ -0,0 +1,101 @@
+import { afterAll, beforeEach, describe, expect, it, vi } from 'vitest'
+import { prisma, createCaller } from '../setup'
+import { createTestUser, cleanupTestData, uid } from '../helpers'
+import { userRouter } from '../../src/server/routers/user'
+import * as emailModule from '../../src/lib/email'
+import type { UserRole } from '@prisma/client'
+
+async function createUserWithRoles(primaryRole: UserRole, rolesArray: UserRole[]) {
+  const id = uid('user')
+  return prisma.user.create({
+    data: {
+      id,
+      email: `${id}@test.local`,
+      name: `Test ${primaryRole}`,
+      role: primaryRole,
+      roles: rolesArray,
+      status: 'ACTIVE',
+    },
+  })
+}
+
+describe('user.bulkUpdateRoles', () => {
+  const userIds: string[] = []
+
+  afterAll(async () => {
+    if (userIds.length > 0) {
+      await prisma.auditLog.deleteMany({ where: { userId: { in: userIds } } })
+      await prisma.decisionAuditLog.deleteMany({ where: { actorId: { in: userIds } } })
+      await prisma.user.deleteMany({ where: { id: { in: userIds } } })
+    }
+  })
+
+  beforeEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  it('adds MENTOR role to multiple users and sends one onboarding email each', async () => {
+    const admin = await createTestUser('SUPER_ADMIN')
+    const j1 = await createUserWithRoles('JURY_MEMBER', ['JURY_MEMBER'])
+    const j2 = await createUserWithRoles('JURY_MEMBER', ['JURY_MEMBER'])
+    userIds.push(admin.id, j1.id, j2.id)
+
+    const sendSpy = vi.spyOn(emailModule, 'sendMentorOnboardingEmail').mockResolvedValue()
+
+    const caller = createCaller(userRouter, { id: admin.id, email: admin.email, role: 'SUPER_ADMIN' })
+    const result = await caller.bulkUpdateRoles({
+      userIds: [j1.id, j2.id],
+      addRole: 'MENTOR',
+    })
+
+    expect(result.updated).toBe(2)
+    expect(result.alreadyHadRole).toBe(0)
+    expect(sendSpy).toHaveBeenCalledTimes(2)
+
+    const updatedJ1 = await prisma.user.findUniqueOrThrow({ where: { id: j1.id } })
+    expect(updatedJ1.roles).toContain('MENTOR')
+    expect(updatedJ1.roles).toContain('JURY_MEMBER')
+    expect(updatedJ1.mentorOnboardingSentAt).not.toBeNull()
+  })
+
+  it('is idempotent — second call with same input does NOT resend onboarding email', async () => {
+    const admin = await createTestUser('SUPER_ADMIN')
+    const j = await createUserWithRoles('JURY_MEMBER', ['JURY_MEMBER'])
+    userIds.push(admin.id, j.id)
+
+    const sendSpy = vi.spyOn(emailModule, 'sendMentorOnboardingEmail').mockResolvedValue()
+
+    const caller = createCaller(userRouter, { id: admin.id, email: admin.email, role: 'SUPER_ADMIN' })
+    await caller.bulkUpdateRoles({ userIds: [j.id], addRole: 'MENTOR' })
+    expect(sendSpy).toHaveBeenCalledTimes(1)
+
+    const second = await caller.bulkUpdateRoles({ userIds: [j.id], addRole: 'MENTOR' })
+    expect(sendSpy).toHaveBeenCalledTimes(1) // still 1
+    expect(second.updated).toBe(0)
+    expect(second.alreadyHadRole).toBe(1)
+  })
+
+  it('removeRole strips the role; does not affect onboarding stamp', async () => {
+    const admin = await createTestUser('SUPER_ADMIN')
+    const m = await createUserWithRoles('MENTOR', ['MENTOR', 'JURY_MEMBER'])
+    userIds.push(admin.id, m.id)
+
+    const caller = createCaller(userRouter, { id: admin.id, email: admin.email, role: 'SUPER_ADMIN' })
+    const result = await caller.bulkUpdateRoles({ userIds: [m.id], removeRole: 'JURY_MEMBER' })
+
+    expect(result.updated).toBe(1)
+    const updated = await prisma.user.findUniqueOrThrow({ where: { id: m.id } })
+    expect(updated.roles).toEqual(['MENTOR'])
+  })
+
+  it('refuses to remove SUPER_ADMIN from self', async () => {
+    const admin = await createTestUser('SUPER_ADMIN')
+    userIds.push(admin.id)
+    await prisma.user.update({ where: { id: admin.id }, data: { roles: ['SUPER_ADMIN'] } })
+
+    const caller = createCaller(userRouter, { id: admin.id, email: admin.email, role: 'SUPER_ADMIN' })
+    await expect(
+      caller.bulkUpdateRoles({ userIds: [admin.id], removeRole: 'SUPER_ADMIN' }),
+    ).rejects.toThrow(/cannot remove SUPER_ADMIN from self|self-demote/i)
+  })
+})