diff --git a/server/auth.integration.test.ts b/server/auth.integration.test.ts
index 2c177cc1..cf9f7c4d 100644
--- a/server/auth.integration.test.ts
+++ b/server/auth.integration.test.ts
@@ -56,13 +56,36 @@ describe('registration gate — open mode', () => {
     expect(res.status).toBe(200)
   })
 
-  it('second user can register when auth_signup_mode is explicitly open', async () => {
+  it('second user can register when auth_signup_mode is explicitly open and instance has Pro license', async () => {
     const ctx = await createTestApp()
+    // Simulate Pro license with open_registration feature
+    const cert = JSON.stringify({
+      account_id: 'test',
+      instance_id: 'test',
+      plan: 'pro',
+      features: ['open_registration'],
+      issued_at: new Date().toISOString(),
+      expires_at: new Date(Date.now() + 86400 * 1000).toISOString(),
+    })
+    await ctx.db.insert(schema.licenseBinding).values({
+      id: 1,
+      instanceId: 'test',
+      refreshToken: 'test-token',
+      cachedCert: cert,
+    })
     await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'open' })
     await signUp(ctx, 'first@example.com')
     const res = await signUp(ctx, 'second@example.com')
     expect(res.status).toBe(200)
   })
+
+  it('second user is rejected when auth_signup_mode is explicitly open but instance has no Pro license', async () => {
+    const ctx = await createTestApp()
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'open' })
+    await signUp(ctx, 'first@example.com')
+    const res = await signUp(ctx, 'second@example.com')
+    expect(res.status).toBe(422)
+  })
 })
 
 describe('registration gate — closed mode', () => {
diff --git a/server/auth.ts b/server/auth.ts
index 950b8b8b..818f8d91 100644
--- a/server/auth.ts
+++ b/server/auth.ts
@@ -20,6 +20,7 @@ import type { Database } from './platform/interface'
 import { sendEmail } from './services/email'
 import { redeemInviteCode, validateInviteCode } from './services/invite'
 import { findPersonalOrg } from './services/org'
+import { getEffectiveSignupMode } from './services/signup-mode-guard'
 
 // better-auth's default password hasher is pure-JS scrypt from @noble/hashes,
 // which blows past Cloudflare Workers' CPU budget and triggers error 1102.
@@ -75,16 +76,6 @@ function buildDynamicSocialProviders(db: Database) {
   return providers
 }
 
-async function getSignupMode(db: Database): Promise<SignupMode> {
-  const rows = await db
-    .select({ value: systemOptions.value })
-    .from(systemOptions)
-    .where(eq(systemOptions.key, 'auth_signup_mode'))
-  const raw = rows[0]?.value
-  if (raw === SignupMode.INVITE_ONLY || raw === SignupMode.CLOSED) return raw
-  return SignupMode.OPEN
-}
-
 async function isEmailConfigured(db: Database): Promise<boolean> {
   const rows = await db
     .select({ value: systemOptions.value })
@@ -213,7 +204,7 @@ export async function createAuth(db: Database, secret: string, baseURL?: string,
 
             // Registration gate: skip for the very first user so bootstrap works
             if (!firstUser) {
-              const mode = await getSignupMode(db)
+              const mode = await getEffectiveSignupMode(db)
               if (mode === SignupMode.CLOSED) {
                 throw new Error('Registration is currently closed')
               }
@@ -248,7 +239,7 @@ export async function createAuth(db: Database, secret: string, baseURL?: string,
           },
           after: async (user, context) => {
             // Redeem invite code after user is created (user.id is now available)
-            const mode = await getSignupMode(db)
+            const mode = await getEffectiveSignupMode(db)
             if (mode === SignupMode.INVITE_ONLY) {
               const inviteCode = (context?.body as { inviteCode?: string })?.inviteCode
               if (inviteCode) {
diff --git a/server/routes/system.ts b/server/routes/system.ts
index e3e7cdef..ffb5fd04 100644
--- a/server/routes/system.ts
+++ b/server/routes/system.ts
@@ -2,7 +2,9 @@ import { zValidator } from '@hono/zod-validator'
 import { eq } from 'drizzle-orm'
 import { Hono } from 'hono'
 import { z } from 'zod'
+import { SignupMode } from '../../shared/constants'
 import { systemOptions } from '../db/schema'
+import { hasFeature, loadBindingState } from '../licensing/has-feature'
 import { requireAdmin } from '../middleware/auth'
 import type { Env } from '../middleware/platform'
 
@@ -36,6 +38,17 @@ const app = new Hono<Env>()
     const db = c.get('platform').db
     const key = c.req.param('key')
     const body = c.req.valid('json')
+
+    if (key === 'auth_signup_mode' && body.value === SignupMode.OPEN) {
+      const state = await loadBindingState(db)
+      if (!hasFeature('open_registration', state)) {
+        return c.json(
+          { error: 'feature_not_available', feature: 'open_registration', upgrade_url: '/settings/billing' },
+          402,
+        )
+      }
+    }
+
     const existing = await db
       .select({ key: systemOptions.key, public: systemOptions.public })
       .from(systemOptions)
diff --git a/server/services/signup-mode-guard.integration.test.ts b/server/services/signup-mode-guard.integration.test.ts
new file mode 100644
index 00000000..51087928
--- /dev/null
+++ b/server/services/signup-mode-guard.integration.test.ts
@@ -0,0 +1,218 @@
+import { describe, expect, it } from 'vitest'
+import * as schema from '../db/schema.js'
+import { createTestApp } from '../test/setup.js'
+import { generateInviteCodes } from './invite.js'
+
+type TestCtx = Awaited<ReturnType<typeof createTestApp>>
+
+async function signUp(ctx: TestCtx, email: string, extra?: Record<string, unknown>) {
+  return ctx.app.request('/api/auth/sign-up/email', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ name: 'Test User', email, password: 'password123456', ...extra }),
+  })
+}
+
+async function seedProLicense(ctx: TestCtx, features: string[] = ['open_registration']) {
+  const cert = JSON.stringify({
+    account_id: 'test-account',
+    instance_id: 'test-instance',
+    plan: 'pro',
+    features,
+    issued_at: new Date().toISOString(),
+    expires_at: new Date(Date.now() + 86400 * 1000).toISOString(),
+  })
+  await ctx.db.insert(schema.licenseBinding).values({
+    id: 1,
+    instanceId: 'test-instance',
+    refreshToken: 'test-refresh-token',
+    cachedCert: cert,
+  })
+}
+
+async function seedFirstUser(ctx: TestCtx) {
+  await signUp(ctx, 'admin@example.com')
+}
+
+// ─── open mode × Pro ─────────────────────────────────────────────────────────
+
+describe('open mode (Pro instance)', () => {
+  it('second user can register without invite code', async () => {
+    const ctx = await createTestApp()
+    await seedProLicense(ctx)
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'open' })
+    await seedFirstUser(ctx)
+    const res = await signUp(ctx, 'second@example.com')
+    expect(res.status).toBe(200)
+  })
+
+  it('third user can also register without invite code', async () => {
+    const ctx = await createTestApp()
+    await seedProLicense(ctx)
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'open' })
+    await seedFirstUser(ctx)
+    await signUp(ctx, 'second@example.com')
+    const res = await signUp(ctx, 'third@example.com')
+    expect(res.status).toBe(200)
+  })
+})
+
+// ─── open mode × non-Pro (retroactive gate) ──────────────────────────────────
+
+describe('open mode (non-Pro instance) — retroactive gate', () => {
+  it('second user is rejected when stored mode is open but instance has no Pro license', async () => {
+    const ctx = await createTestApp()
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'open' })
+    await seedFirstUser(ctx)
+    const res = await signUp(ctx, 'second@example.com')
+    expect(res.status).not.toBe(200)
+  })
+
+  it('rejection returns 422 (same as invite_only behaviour)', async () => {
+    const ctx = await createTestApp()
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'open' })
+    await seedFirstUser(ctx)
+    const res = await signUp(ctx, 'second@example.com')
+    expect(res.status).toBe(422)
+  })
+
+  it('second user can register when a valid invite code is supplied (falls back to invite-only)', async () => {
+    const ctx = await createTestApp()
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'open' })
+    await seedFirstUser(ctx)
+    const [codeRow] = await generateInviteCodes(ctx.db, 'admin-1', 1)
+    const res = await signUp(ctx, 'invited@example.com', { inviteCode: codeRow.code })
+    expect(res.status).toBe(200)
+  })
+})
+
+// ─── invite_only mode × Pro ──────────────────────────────────────────────────
+
+describe('invite_only mode (Pro instance)', () => {
+  it('second user is rejected without invite code', async () => {
+    const ctx = await createTestApp()
+    await seedProLicense(ctx)
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'invite_only' })
+    await seedFirstUser(ctx)
+    const res = await signUp(ctx, 'noinvite@example.com')
+    expect(res.status).not.toBe(200)
+  })
+
+  it('second user can register with valid invite code', async () => {
+    const ctx = await createTestApp()
+    await seedProLicense(ctx)
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'invite_only' })
+    await seedFirstUser(ctx)
+    const [codeRow] = await generateInviteCodes(ctx.db, 'admin-1', 1)
+    const res = await signUp(ctx, 'invited@example.com', { inviteCode: codeRow.code })
+    expect(res.status).toBe(200)
+  })
+})
+
+// ─── invite_only mode × non-Pro ──────────────────────────────────────────────
+
+describe('invite_only mode (non-Pro instance)', () => {
+  it('second user is rejected without invite code', async () => {
+    const ctx = await createTestApp()
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'invite_only' })
+    await seedFirstUser(ctx)
+    const res = await signUp(ctx, 'noinvite@example.com')
+    expect(res.status).not.toBe(200)
+  })
+
+  it('second user can register with valid invite code', async () => {
+    const ctx = await createTestApp()
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'invite_only' })
+    await seedFirstUser(ctx)
+    const [codeRow] = await generateInviteCodes(ctx.db, 'admin-1', 1)
+    const res = await signUp(ctx, 'invited@example.com', { inviteCode: codeRow.code })
+    expect(res.status).toBe(200)
+  })
+})
+
+// ─── closed mode × Pro ───────────────────────────────────────────────────────
+
+describe('closed mode (Pro instance)', () => {
+  it('second user is rejected', async () => {
+    const ctx = await createTestApp()
+    await seedProLicense(ctx)
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'closed' })
+    await seedFirstUser(ctx)
+    const res = await signUp(ctx, 'second@example.com')
+    expect(res.status).not.toBe(200)
+  })
+})
+
+// ─── closed mode × non-Pro ───────────────────────────────────────────────────
+
+describe('closed mode (non-Pro instance)', () => {
+  it('second user is rejected', async () => {
+    const ctx = await createTestApp()
+    await ctx.db.insert(schema.systemOptions).values({ key: 'auth_signup_mode', value: 'closed' })
+    await seedFirstUser(ctx)
+    const res = await signUp(ctx, 'second@example.com')
+    expect(res.status).not.toBe(200)
+  })
+})
+
+// ─── PUT /api/system/options/auth_signup_mode ─────────────────────────────────
+
+describe('PUT auth_signup_mode via admin API', () => {
+  async function adminHeaders(ctx: TestCtx) {
+    await signUp(ctx, 'admin@example.com')
+    const signInRes = await ctx.app.request('/api/auth/sign-in/email', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ email: 'admin@example.com', password: 'password123456' }),
+    })
+    return { Cookie: signInRes.headers.getSetCookie().join('; ') }
+  }
+
+  async function putSignupMode(ctx: TestCtx, headers: Record<string, string>, value: string) {
+    return ctx.app.request('/api/system/options/auth_signup_mode', {
+      method: 'PUT',
+      headers: { ...headers, 'Content-Type': 'application/json' },
+      body: JSON.stringify({ value, public: true }),
+    })
+  }
+
+  it('setting open without Pro returns 402', async () => {
+    const ctx = await createTestApp()
+    const headers = await adminHeaders(ctx)
+    const res = await putSignupMode(ctx, headers, 'open')
+    expect(res.status).toBe(402)
+    const body = (await res.json()) as { error: string; feature: string }
+    expect(body.error).toBe('feature_not_available')
+    expect(body.feature).toBe('open_registration')
+  })
+
+  it('setting open with Pro succeeds', async () => {
+    const ctx = await createTestApp()
+    await seedProLicense(ctx)
+    const headers = await adminHeaders(ctx)
+    const res = await putSignupMode(ctx, headers, 'open')
+    expect(res.status).toBe(201)
+  })
+
+  it('setting invite_only without Pro succeeds', async () => {
+    const ctx = await createTestApp()
+    const headers = await adminHeaders(ctx)
+    const res = await putSignupMode(ctx, headers, 'invite_only')
+    expect(res.status).toBe(201)
+  })
+
+  it('setting closed without Pro succeeds', async () => {
+    const ctx = await createTestApp()
+    const headers = await adminHeaders(ctx)
+    const res = await putSignupMode(ctx, headers, 'closed')
+    expect(res.status).toBe(201)
+  })
+
+  it('setting invite_only with Pro succeeds', async () => {
+    const ctx = await createTestApp()
+    await seedProLicense(ctx)
+    const headers = await adminHeaders(ctx)
+    const res = await putSignupMode(ctx, headers, 'invite_only')
+    expect(res.status).toBe(201)
+  })
+})
diff --git a/server/services/signup-mode-guard.ts b/server/services/signup-mode-guard.ts
new file mode 100644
index 00000000..71fe4b7e
--- /dev/null
+++ b/server/services/signup-mode-guard.ts
@@ -0,0 +1,28 @@
+import { eq } from 'drizzle-orm'
+import { SignupMode } from '../../shared/constants'
+import { systemOptions } from '../db/schema'
+import { hasFeature, loadBindingState } from '../licensing/has-feature'
+import type { Database } from '../platform/interface'
+
+/**
+ * Returns the effective signup mode.
+ *
+ * Rule: `open` requires the `open_registration` Pro feature. Without it the
+ * effective mode falls back to `invite-only`. All other stored values
+ * (invite_only, closed) are returned unchanged. Unknown/empty values retain
+ * the existing default-to-open behaviour and are not subject to the Pro check.
+ */
+export async function getEffectiveSignupMode(db: Database): Promise<SignupMode> {
+  const rows = await db
+    .select({ value: systemOptions.value })
+    .from(systemOptions)
+    .where(eq(systemOptions.key, 'auth_signup_mode'))
+  const raw = rows[0]?.value
+
+  if (raw === SignupMode.INVITE_ONLY || raw === SignupMode.CLOSED) return raw
+  if (raw !== SignupMode.OPEN) return SignupMode.OPEN // unknown/empty → open (existing behaviour)
+
+  // Stored value is explicitly 'open' — gate behind Pro feature
+  const state = await loadBindingState(db)
+  return hasFeature('open_registration', state) ? SignupMode.OPEN : SignupMode.INVITE_ONLY
+}
diff --git a/src/components/admin/registration-mode-section.tsx b/src/components/admin/registration-mode-section.tsx
index c52e1f99..8e01a086 100644
--- a/src/components/admin/registration-mode-section.tsx
+++ b/src/components/admin/registration-mode-section.tsx
@@ -1,9 +1,14 @@
 import { SignupMode } from '@shared/constants'
 import { useMutation, useQueryClient } from '@tanstack/react-query'
+import { useState } from 'react'
 import { useTranslation } from 'react-i18next'
 import { toast } from 'sonner'
+import { ProBadge } from '@/components/ProBadge'
+import { UpgradeHint } from '@/components/UpgradeHint'
+import { Dialog, DialogContent } from '@/components/ui/dialog'
 import { Label } from '@/components/ui/label'
 import { siteOptionsQueryKey, useSiteOptions } from '@/hooks/use-site-options'
+import { useEntitlement } from '@/hooks/useEntitlement'
 import { setSystemOption } from '@/lib/api'
 
 const modes = [
@@ -24,6 +29,9 @@ export function RegistrationModeSection() {
   const { t } = useTranslation()
   const queryClient = useQueryClient()
   const { authSignupMode } = useSiteOptions()
+  const { hasFeature } = useEntitlement()
+  const [upgradeOpen, setUpgradeOpen] = useState(false)
+  const hasOpenReg = hasFeature('open_registration')
 
   const mutation = useMutation({
     mutationFn: (mode: string) => setSystemOption('auth_signup_mode', mode, true),
@@ -34,28 +42,53 @@ export function RegistrationModeSection() {
     onError: (err) => toast.error(err.message),
   })
 
+  function handleModeChange(mode: string) {
+    if (mode === SignupMode.OPEN && !hasOpenReg) {
+      setUpgradeOpen(true)
+      return
+    }
+    mutation.mutate(mode)
+  }
+
   return (
-    <div className="space-y-4 rounded-md border p-4">
-      <h3 className="text-sm font-medium text-muted-foreground">{t('admin.auth.registrationSection')}</h3>
-      <div className="space-y-3">
-        {modes.map((mode) => (
-          <Label key={mode.value} className="flex items-start gap-3 cursor-pointer">
-            <input
-              type="radio"
-              name="signupMode"
-              value={mode.value}
-              checked={authSignupMode === mode.value}
-              onChange={() => mutation.mutate(mode.value)}
-              disabled={mutation.isPending}
-              className="mt-1"
-            />
-            <div>
-              <div className="font-medium">{t(mode.labelKey)}</div>
-              <div className="text-xs text-muted-foreground">{t(mode.descKey)}</div>
-            </div>
-          </Label>
-        ))}
+    <>
+      <div className="space-y-4 rounded-md border p-4">
+        <h3 className="text-sm font-medium text-muted-foreground">{t('admin.auth.registrationSection')}</h3>
+        <div className="space-y-3">
+          {modes.map((mode) => {
+            const isOpenGated = mode.value === SignupMode.OPEN && !hasOpenReg
+            return (
+              <Label
+                key={mode.value}
+                className="flex items-start gap-3 cursor-pointer"
+                onClick={isOpenGated ? () => setUpgradeOpen(true) : undefined}
+              >
+                <input
+                  type="radio"
+                  name="signupMode"
+                  value={mode.value}
+                  checked={authSignupMode === mode.value}
+                  onChange={() => handleModeChange(mode.value)}
+                  disabled={mutation.isPending || isOpenGated}
+                  className="mt-1"
+                />
+                <div>
+                  <div className="font-medium flex items-center gap-2">
+                    {t(mode.labelKey)}
+                    {isOpenGated && <ProBadge />}
+                  </div>
+                  <div className="text-xs text-muted-foreground">{t(mode.descKey)}</div>
+                </div>
+              </Label>
+            )
+          })}
+        </div>
       </div>
-    </div>
+      <Dialog open={upgradeOpen} onOpenChange={setUpgradeOpen}>
+        <DialogContent>
+          <UpgradeHint feature="open_registration" />
+        </DialogContent>
+      </Dialog>
+    </>
   )
 }