diff --git a/apps/portal-bff/src/admin/admin-role.guard.spec.ts b/apps/portal-bff/src/admin/admin-role.guard.spec.ts
new file mode 100644
index 0000000..fd1880d
--- /dev/null
+++ b/apps/portal-bff/src/admin/admin-role.guard.spec.ts
@@ -0,0 +1,178 @@
+import { ExecutionContext, ForbiddenException, UnauthorizedException } from '@nestjs/common';
+import type { Request } from 'express';
+import { AuditWriter } from '../audit/audit.service';
+import { ADMIN_ROLE, AdminRoleGuard } from './admin-role.guard';
+
+interface SessionStub {
+  user?: {
+    oid: string;
+    tid: string;
+    username: string;
+    displayName: string;
+    amr: readonly string[];
+    roles: readonly string[];
+  };
+}
+
+function makeRequest(opts: {
+  session?: SessionStub;
+  method?: string;
+  originalUrl?: string;
+}): Request {
+  return {
+    session: opts.session,
+    method: opts.method ?? 'GET',
+    originalUrl: opts.originalUrl ?? '/api/admin/me',
+  } as unknown as Request;
+}
+
+function makeContext(req: Request): ExecutionContext {
+  return {
+    switchToHttp: () => ({ getRequest: <T>() => req as T }),
+  } as unknown as ExecutionContext;
+}
+
+function makeAuditStub(): jest.Mocked<Pick<AuditWriter, 'adminAccessDenied'>> {
+  return {
+    adminAccessDenied: jest.fn().mockResolvedValue(undefined),
+  };
+}
+
+describe('AdminRoleGuard', () => {
+  it('throws 401 when the request has no session at all', async () => {
+    const audit = makeAuditStub();
+    const guard = new AdminRoleGuard(audit as unknown as AuditWriter);
+    const ctx = makeContext(makeRequest({ session: undefined }));
+    await expect(guard.canActivate(ctx)).rejects.toBeInstanceOf(UnauthorizedException);
+    expect(audit.adminAccessDenied).not.toHaveBeenCalled();
+  });
+
+  it('throws 401 when the session exists but no user is bound to it', async () => {
+    const audit = makeAuditStub();
+    const guard = new AdminRoleGuard(audit as unknown as AuditWriter);
+    const ctx = makeContext(makeRequest({ session: {} }));
+    await expect(guard.canActivate(ctx)).rejects.toBeInstanceOf(UnauthorizedException);
+    expect(audit.adminAccessDenied).not.toHaveBeenCalled();
+  });
+
+  it('throws 403 + records admin.access_denied when the user has no roles at all', async () => {
+    const audit = makeAuditStub();
+    const guard = new AdminRoleGuard(audit as unknown as AuditWriter);
+    const ctx = makeContext(
+      makeRequest({
+        session: {
+          user: {
+            oid: 'user-oid',
+            tid: 't',
+            username: 'jane@example',
+            displayName: 'Jane',
+            amr: ['pwd'],
+            roles: [],
+          },
+        },
+        method: 'GET',
+        originalUrl: '/api/admin/me',
+      }),
+    );
+    await expect(guard.canActivate(ctx)).rejects.toBeInstanceOf(ForbiddenException);
+    expect(audit.adminAccessDenied).toHaveBeenCalledWith({
+      actor: { oid: 'user-oid' },
+      attemptedRoute: 'GET /api/admin/me',
+      rolesHeld: [],
+    });
+  });
+
+  it('throws 403 + records admin.access_denied when the user has unrelated roles only', async () => {
+    const audit = makeAuditStub();
+    const guard = new AdminRoleGuard(audit as unknown as AuditWriter);
+    const ctx = makeContext(
+      makeRequest({
+        session: {
+          user: {
+            oid: 'user-oid',
+            tid: 't',
+            username: 'jane@example',
+            displayName: 'Jane',
+            amr: ['pwd'],
+            roles: ['editor', 'auditor'],
+          },
+        },
+        method: 'POST',
+        originalUrl: '/api/admin/cms/pages',
+      }),
+    );
+    await expect(guard.canActivate(ctx)).rejects.toBeInstanceOf(ForbiddenException);
+    expect(audit.adminAccessDenied).toHaveBeenCalledWith({
+      actor: { oid: 'user-oid' },
+      attemptedRoute: 'POST /api/admin/cms/pages',
+      rolesHeld: ['editor', 'auditor'],
+    });
+  });
+
+  it('returns true and does not audit when the user has the admin role', async () => {
+    const audit = makeAuditStub();
+    const guard = new AdminRoleGuard(audit as unknown as AuditWriter);
+    const ctx = makeContext(
+      makeRequest({
+        session: {
+          user: {
+            oid: 'user-oid',
+            tid: 't',
+            username: 'jane@example',
+            displayName: 'Jane',
+            amr: ['pwd', 'mfa'],
+            roles: [ADMIN_ROLE],
+          },
+        },
+      }),
+    );
+    await expect(guard.canActivate(ctx)).resolves.toBe(true);
+    expect(audit.adminAccessDenied).not.toHaveBeenCalled();
+  });
+
+  it('returns true when admin is one of several roles', async () => {
+    const audit = makeAuditStub();
+    const guard = new AdminRoleGuard(audit as unknown as AuditWriter);
+    const ctx = makeContext(
+      makeRequest({
+        session: {
+          user: {
+            oid: 'user-oid',
+            tid: 't',
+            username: 'jane@example',
+            displayName: 'Jane',
+            amr: ['pwd', 'mfa'],
+            roles: ['editor', ADMIN_ROLE, 'auditor'],
+          },
+        },
+      }),
+    );
+    await expect(guard.canActivate(ctx)).resolves.toBe(true);
+  });
+
+  it('propagates audit write failures (no audit ⇒ no action, per ADR-0013)', async () => {
+    const audit = {
+      adminAccessDenied: jest.fn().mockRejectedValue(new Error('audit_writer denied')),
+    };
+    const guard = new AdminRoleGuard(audit as unknown as AuditWriter);
+    const ctx = makeContext(
+      makeRequest({
+        session: {
+          user: {
+            oid: 'user-oid',
+            tid: 't',
+            username: 'jane@example',
+            displayName: 'Jane',
+            amr: ['pwd'],
+            roles: [],
+          },
+        },
+      }),
+    );
+    // The 403 path goes through audit first; if audit throws, the
+    // guard surfaces the underlying error rather than the
+    // ForbiddenException — the caller sees a 500 and the audit
+    // invariant is preserved.
+    await expect(guard.canActivate(ctx)).rejects.toThrow('audit_writer denied');
+  });
+});
diff --git a/apps/portal-bff/src/admin/admin-role.guard.ts b/apps/portal-bff/src/admin/admin-role.guard.ts
new file mode 100644
index 0000000..71ac5c1
--- /dev/null
+++ b/apps/portal-bff/src/admin/admin-role.guard.ts
@@ -0,0 +1,68 @@
+import {
+  CanActivate,
+  ExecutionContext,
+  ForbiddenException,
+  Injectable,
+  UnauthorizedException,
+} from '@nestjs/common';
+import type { Request } from 'express';
+import { AuditWriter } from '../audit/audit.service';
+
+/**
+ * Single Entra app role that gates the entire `/api/admin/*` surface
+ * per ADR-0020 §"Auth — `admin` role claim". The value matches the
+ * `value` field declared on the app role in the Entra app
+ * registration manifest. Defined as a constant so spec assertions
+ * can refer to the same source of truth.
+ */
+export const ADMIN_ROLE = 'admin';
+
+/**
+ * `AdminRoleGuard` — enforces ADR-0020's role-based admin gate.
+ *
+ * Contract
+ * --------
+ * - **No session → 401.** The user is not authenticated at all; the
+ *   SPA should kick off the login flow. We do not audit anonymous
+ *   probes here because the route family is not a target — any
+ *   unauthenticated 401 is normal traffic the absolute-timeout
+ *   middleware would surface anyway.
+ *
+ * - **Session but missing `admin` role → 403 + audit.** The user
+ *   *is* authenticated; they just are not authorised for admin.
+ *   This is the privilege-escalation attempt audit signal — every
+ *   denial lands in `audit.events` with `outcome=denied`, the
+ *   attempted route as `subject`, and the roles the user did hold
+ *   in the payload. Per ADR-0013 §"Blocking writes": no audit ⇒ no
+ *   action — if the audit write fails, the request fails too
+ *   (consistent with the existing audit call sites in
+ *   `AuthController`).
+ *
+ * - **Session with `admin` role → pass through.** Downstream
+ *   controllers see `req.session.user` populated and can rely on
+ *   the role check having happened.
+ */
+@Injectable()
+export class AdminRoleGuard implements CanActivate {
+  constructor(private readonly audit: AuditWriter) {}
+
+  async canActivate(context: ExecutionContext): Promise<boolean> {
+    const req = context.switchToHttp().getRequest<Request>();
+    const user = req.session?.user;
+
+    if (!user) {
+      throw new UnauthorizedException();
+    }
+
+    if (!user.roles.includes(ADMIN_ROLE)) {
+      await this.audit.adminAccessDenied({
+        actor: { oid: user.oid },
+        attemptedRoute: `${req.method} ${req.originalUrl}`,
+        rolesHeld: user.roles,
+      });
+      throw new ForbiddenException();
+    }
+
+    return true;
+  }
+}
diff --git a/apps/portal-bff/src/admin/admin.controller.spec.ts b/apps/portal-bff/src/admin/admin.controller.spec.ts
new file mode 100644
index 0000000..6e5a282
--- /dev/null
+++ b/apps/portal-bff/src/admin/admin.controller.spec.ts
@@ -0,0 +1,51 @@
+import type { Request } from 'express';
+import { AdminController } from './admin.controller';
+
+function makeReq(user: {
+  oid: string;
+  tid: string;
+  username: string;
+  displayName: string;
+  amr: readonly string[];
+  roles: readonly string[];
+}): Request {
+  return { session: { user } } as unknown as Request;
+}
+
+describe('AdminController', () => {
+  it('GET /me returns the public payload of the admin user, including roles', () => {
+    const controller = new AdminController();
+    const res = controller.me(
+      makeReq({
+        oid: 'admin-oid',
+        tid: 'tenant-1',
+        username: 'admin@apf.example',
+        displayName: 'Admin Smith',
+        amr: ['pwd', 'mfa'],
+        roles: ['admin'],
+      }),
+    );
+    expect(res).toEqual({
+      oid: 'admin-oid',
+      tid: 'tenant-1',
+      username: 'admin@apf.example',
+      displayName: 'Admin Smith',
+      roles: ['admin'],
+    });
+  });
+
+  it('does not leak server-internal claims (`amr`) to the response', () => {
+    const controller = new AdminController();
+    const res = controller.me(
+      makeReq({
+        oid: 'admin-oid',
+        tid: 'tenant-1',
+        username: 'admin@apf.example',
+        displayName: 'Admin Smith',
+        amr: ['pwd', 'mfa'],
+        roles: ['admin'],
+      }),
+    );
+    expect(res).not.toHaveProperty('amr');
+  });
+});
diff --git a/apps/portal-bff/src/admin/admin.controller.ts b/apps/portal-bff/src/admin/admin.controller.ts
new file mode 100644
index 0000000..14fdf8d
--- /dev/null
+++ b/apps/portal-bff/src/admin/admin.controller.ts
@@ -0,0 +1,40 @@
+import { Controller, Get, Req } from '@nestjs/common';
+import type { Request } from 'express';
+import { RequireAdmin } from './require-admin.decorator';
+
+/**
+ * `/api/admin/*` controllers per ADR-0020. The `@RequireAdmin()`
+ * class-level decorator gates every route on the Entra `admin` role;
+ * downstream PRs will add per-method `@RequireMfa({ freshness: … })`
+ * for the entry route.
+ *
+ * `GET /api/admin/me` is the self-test endpoint named in ADR-0020's
+ * "Confirmation" §3 ("AdminModule with AdminRoleGuard + first
+ * endpoint (`GET /api/admin/me` returning the session for
+ * self-test)"). It lets ops verify the guard is wired correctly
+ * (curl with no cookies → 401; curl with a non-admin session → 403
+ * with an `admin.access_denied` audit row; curl with an admin
+ * session → 200 + the public user payload) before any real admin
+ * route ships.
+ */
+@Controller('admin')
+@RequireAdmin()
+export class AdminController {
+  @Get('me')
+  me(@Req() req: Request) {
+    // `AdminRoleGuard` has already established that `req.session.user`
+    // is present and has the `admin` role — the handler can read the
+    // field without re-checking. Using the non-null assertion keeps
+    // the dead-branch noise out of the controller while leaving the
+    // safety contract anchored in the guard's spec.
+    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion -- guaranteed by AdminRoleGuard
+    const user = req.session.user!;
+    return {
+      oid: user.oid,
+      tid: user.tid,
+      username: user.username,
+      displayName: user.displayName,
+      roles: user.roles,
+    };
+  }
+}
diff --git a/apps/portal-bff/src/admin/admin.module.ts b/apps/portal-bff/src/admin/admin.module.ts
new file mode 100644
index 0000000..d5c765f
--- /dev/null
+++ b/apps/portal-bff/src/admin/admin.module.ts
@@ -0,0 +1,21 @@
+import { Module } from '@nestjs/common';
+import { AdminController } from './admin.controller';
+import { AdminRoleGuard } from './admin-role.guard';
+
+/**
+ * `AdminModule` — root of the `/api/admin/*` surface per ADR-0020.
+ *
+ * v1 ships the self-test endpoint only (`GET /api/admin/me`). The
+ * functional admin modules (audit log viewer, CMS, menu management,
+ * user list) land as separate sub-modules consumed from here once
+ * the distinct admin session + the audit query endpoint are in
+ * place — see the chantier sequence in `notes/handoff.md`.
+ *
+ * `AuditWriter` (required by `AdminRoleGuard`) is provided globally
+ * by `AuditModule`, so no extra import is needed here.
+ */
+@Module({
+  controllers: [AdminController],
+  providers: [AdminRoleGuard],
+})
+export class AdminModule {}
diff --git a/apps/portal-bff/src/admin/require-admin.decorator.ts b/apps/portal-bff/src/admin/require-admin.decorator.ts
new file mode 100644
index 0000000..2f31ddb
--- /dev/null
+++ b/apps/portal-bff/src/admin/require-admin.decorator.ts
@@ -0,0 +1,20 @@
+import { UseGuards, applyDecorators } from '@nestjs/common';
+import { AdminRoleGuard } from './admin-role.guard';
+
+/**
+ * `@RequireAdmin()` — semantic sugar for `@UseGuards(AdminRoleGuard)`.
+ *
+ * Applied at the controller (class) level so every route in an admin
+ * controller is gated uniformly, or at the method level on the rare
+ * route inside a mixed controller that needs admin. ADR-0020 §"Auth"
+ * names this decorator alongside `@RequireMfa()`; we expose the
+ * decorator API so both gates compose the same way in upcoming PRs.
+ *
+ * Using `applyDecorators` (instead of a one-line factory function)
+ * leaves room for additional decorators in the same wrapper later —
+ * e.g. an `@SetMetadata('rbac:requires', ['admin'])` for OpenAPI /
+ * route-introspection tooling — without changing call sites.
+ */
+export function RequireAdmin(): ClassDecorator & MethodDecorator {
+  return applyDecorators(UseGuards(AdminRoleGuard));
+}
diff --git a/apps/portal-bff/src/app/app.module.ts b/apps/portal-bff/src/app/app.module.ts
index 626a5a3..30d3233 100644
--- a/apps/portal-bff/src/app/app.module.ts
+++ b/apps/portal-bff/src/app/app.module.ts
@@ -4,6 +4,7 @@ import { AppController } from './app.controller';
 import { AppService } from './app.service';
 import { ObservabilityModule } from '../observability/observability.module';
 import { HealthModule } from '../health/health.module';
+import { AdminModule } from '../admin/admin.module';
 import { AuditModule } from '../audit/audit.module';
 import { AuthModule } from '../auth/auth.module';
 import { RedisModule } from '../redis/redis.module';
@@ -20,6 +21,7 @@ import { SessionModule } from '../session/session.module';
     SessionModule,
     SecurityModule,
     HealthModule,
+    AdminModule,
   ],
   controllers: [AppController],
   providers: [AppService],
diff --git a/apps/portal-bff/src/audit/audit.service.spec.ts b/apps/portal-bff/src/audit/audit.service.spec.ts
index 82cd86a..9082ced 100644
--- a/apps/portal-bff/src/audit/audit.service.spec.ts
+++ b/apps/portal-bff/src/audit/audit.service.spec.ts
@@ -315,4 +315,33 @@ describe('AuditWriter — typed event methods', () => {
       );
     });
   });
+
+  describe('adminAccessDenied()', () => {
+    it('records admin.access_denied with outcome=denied, attempted route as subject, and rolesHeld payload', async () => {
+      const { writer, prisma, hashUserId } = await createSubject();
+      await writer.adminAccessDenied({
+        actor: { oid: 'user-oid' },
+        attemptedRoute: 'GET /api/admin/audit',
+        rolesHeld: ['editor'],
+      });
+      expect(hashUserId.hash).toHaveBeenCalledWith('user-oid');
+      const row = extractInsertedRow(prisma);
+      expect(row.eventType).toBe('admin.access_denied');
+      expect(row.audience).toBe('workforce');
+      expect(row.outcome).toBe('denied');
+      expect(row.actorIdHash).toBe('hash(user-oid)');
+      expect(row.subject).toBe('GET /api/admin/audit');
+      expect(row.payloadJson).toBe(JSON.stringify({ rolesHeld: ['editor'] }));
+    });
+
+    it('persists an empty rolesHeld array verbatim (distinguishable from missing payload)', async () => {
+      const { writer, prisma } = await createSubject();
+      await writer.adminAccessDenied({
+        actor: { oid: 'user-oid' },
+        attemptedRoute: 'GET /api/admin/me',
+        rolesHeld: [],
+      });
+      expect(extractInsertedRow(prisma).payloadJson).toBe(JSON.stringify({ rolesHeld: [] }));
+    });
+  });
 });
diff --git a/apps/portal-bff/src/audit/audit.service.ts b/apps/portal-bff/src/audit/audit.service.ts
index 288b664..6ed8046 100644
--- a/apps/portal-bff/src/audit/audit.service.ts
+++ b/apps/portal-bff/src/audit/audit.service.ts
@@ -3,6 +3,7 @@ import { trace } from '@opentelemetry/api';
 import { ClsService } from 'nestjs-cls';
 import { PrismaService } from 'nestjs-prisma';
 import type {
+  AdminAccessDeniedInput,
   AuditEventInput,
   SignInActor,
   SignInFailedInput,
@@ -113,6 +114,26 @@ export class AuditWriter {
     });
   }
 
+  /**
+   * Typed event: `/api/admin/*` request rejected by `AdminRoleGuard`
+   * because the session's `roles` claim does not include `admin`.
+   * Per ADR-0020 §"Auth — same Entra ID … `admin` role claim", every
+   * 403 from the admin surface is captured here with the attempted
+   * route and the roles the user actually held — auditors looking
+   * for privilege-escalation attempts pivot on `subject` (the route)
+   * and `outcome=denied`.
+   */
+  async adminAccessDenied(input: AdminAccessDeniedInput): Promise<void> {
+    await this.recordEvent({
+      eventType: 'admin.access_denied',
+      audience: 'workforce',
+      outcome: 'denied',
+      actorIdHash: this.hashUserId.hash(input.actor.oid),
+      subject: input.attemptedRoute,
+      payload: { rolesHeld: input.rolesHeld },
+    });
+  }
+
   async recordEvent(input: AuditEventInput): Promise<void> {
     const traceId = trace.getActiveSpan()?.spanContext().traceId ?? null;
     const actorIdHash =
diff --git a/apps/portal-bff/src/audit/audit.types.ts b/apps/portal-bff/src/audit/audit.types.ts
index 78bab7f..3f9fefa 100644
--- a/apps/portal-bff/src/audit/audit.types.ts
+++ b/apps/portal-bff/src/audit/audit.types.ts
@@ -82,3 +82,21 @@ export interface SessionExpiredInput {
   /** Age of the session at the moment of expiry, in milliseconds. */
   ageMs: number;
 }
+
+export interface AdminAccessDeniedInput {
+  actor: { oid: string };
+  /**
+   * Canonical "{METHOD} {originalUrl}" of the rejected request, e.g.
+   * `GET /api/admin/audit`. Persisted into the audit row's `subject`
+   * column so auditors can pivot on it without joining anything.
+   */
+  attemptedRoute: string;
+  /**
+   * The roles the user *did* hold at the moment of denial — empty
+   * array when no app role was assigned. Stored in the payload so
+   * an auditor can tell apart "tried admin while logged in as a
+   * regular user" from "tried admin with an unrelated role like
+   * `auditor` (future)" without parsing the deny pattern.
+   */
+  rolesHeld: readonly string[];
+}