diff --git a/src/app/signup/actions.ts b/src/app/signup/actions.ts
index 72055f9..7c38d10 100644
--- a/src/app/signup/actions.ts
+++ b/src/app/signup/actions.ts
@@ -4,7 +4,7 @@ import { revalidatePath } from 'next/cache'
 import { redirect } from 'next/navigation'
 import { AuthError } from '@supabase/supabase-js'
 
-import { createClient } from '@/utils/supabase/server'
+import { createClient, createServiceClient } from '@/utils/supabase/server'
 import { validateEmail, validatePassword } from '@/utils/auth'
 
 function getErrorMessage(error: AuthError): string {
@@ -32,11 +32,13 @@ export interface AuthActionState {
 
 export async function signup(state: AuthActionState, payload: FormData) {
   const supabase = await createClient()
+  const service = await createServiceClient()
 
   const data = {
     email: payload.get('email') as string,
     password: payload.get('password') as string,
   }
+  const inviteCode = (payload.get('inviteCode') as string | null)?.trim() || ''
 
   const { error: emailError } = validateEmail(data.email);
   if (emailError) {
@@ -48,12 +50,48 @@ export async function signup(state: AuthActionState, payload: FormData) {
     return { error: passwordError };
   }
 
-  const { error } = await supabase.auth.signUp(data)
+  if (!inviteCode) {
+    return { error: 'An invite code is required to sign up.' }
+  }
+
+  // Pre-check: ensure invite exists and is unused before attempting signup
+  const { data: availableInvite, error: inviteCheckError } = await service
+    .from('invite_codes')
+    .select('code')
+    .eq('code', inviteCode)
+    .is('used_by', null)
+    .maybeSingle()
+
+  if (inviteCheckError || !availableInvite) {
+    return { error: 'Invalid or already used invite code.' }
+  }
+
+  const { data: signUpResult, error } = await supabase.auth.signUp(data)
 
   if (error) {
     return { error: getErrorMessage(error) };
   }
 
+  const userId = signUpResult.user?.id || null
+  // Finalize: set used_by to the new user id iff still unused (atomic)
+  const { data: finalized, error: finalizeError } = await service
+    .from('invite_codes')
+    .update({ used_by: userId ?? null })
+    .eq('code', inviteCode)
+    .is('used_by', null)
+    .select('code')
+    .maybeSingle()
+
+  if (finalizeError || !finalized) {
+    // The code claim could not be finalized (race). Roll back user creation.
+    if (userId) {
+      try {
+        await service.auth.admin.deleteUser(userId)
+      } catch {}
+    }
+    return { error: 'Invite code is no longer available. Please try again.' }
+  }
+
   revalidatePath('/', 'layout');
   const redirectTo = (payload.get('redirectTo') as string | null) || null
   const isValidInternalPath = redirectTo && redirectTo.startsWith('/') && !redirectTo.startsWith('//')
diff --git a/src/components/Auth/SignupForm.tsx b/src/components/Auth/SignupForm.tsx
index 646e6ab..10296a4 100644
--- a/src/components/Auth/SignupForm.tsx
+++ b/src/components/Auth/SignupForm.tsx
@@ -11,13 +11,14 @@ export default function SignupForm() {
   const [email, setEmail] = React.useState("");
   const [password, setPassword] = React.useState("");
   const [confirm, setConfirm] = React.useState("");
+  const [invite, setInvite] = React.useState<string>("");
   const [showPassword, setShowPassword] = React.useState(false);
   const [emailError, setEmailError] = React.useState<string | null>(null);
   const [passwordError, setPasswordError] = React.useState<string | null>(null);
 
-  const [state, formAction] = useActionState<AuthActionState, FormData>(signup, { error: null });
+  const [state, formAction, isPending] = useActionState<AuthActionState, FormData>(signup, { error: null });
   const passwordsMatch = password === confirm;
-  const isValid = !emailError && !passwordError && passwordsMatch;
+  const isValid = !emailError && !passwordError && passwordsMatch && Boolean(invite);
 
   useEffect(() => {
     const { error } = validateEmail(email);
@@ -31,16 +32,44 @@ export default function SignupForm() {
 
   const redirectTo = searchParams.get("redirectTo");
 
+  useEffect(() => {
+    const inviteFromParams = searchParams.get("invite") || "";
+    if (inviteFromParams) {
+      setInvite(inviteFromParams);
+    }
+  }, []);
+
   return (
     <form className="grid gap-5 group">
       {redirectTo && (
         <input type="hidden" name="redirectTo" value={redirectTo} />
       )}
-      {(state?.error) && (
+      {(state?.error && !isPending) && (
         <div className="rounded-md bg-red-500/10 ring-1 ring-red-600/40 px-3 py-2 text-sm text-red-300">
           {state?.error}
         </div>
       )}
+      <div className="grid gap-2">
+        <label htmlFor="inviteCode" className="text-sm text-foreground/80">Invite code</label>
+        <input
+          id="inviteCode"
+          name="inviteCode"
+          type="text"
+          value={invite}
+          onChange={(e) => setInvite(e.target.value)}
+          placeholder="Enter your invite code"
+          className={`h-11 rounded-md bg-[var(--surface-2)] px-3 text-sm ring-1 ring-inset ring-[var(--border)] focus:outline-none focus:ring-2 focus:ring-[var(--ring)] ${
+            invite ? "bg-[var(--surface-2)] ring-[var(--border)]" : "bg-[var(--surface-2)] ring-[var(--border)]"
+          }`}
+          required
+          inputMode="text"
+          autoComplete="off"
+          spellCheck={false}
+        />
+        {!invite && (
+          <span className="text-xs text-foreground/60">An invite code is required to create an account.</span>
+        )}
+      </div>
       <div className="grid gap-2">
         <label htmlFor="email" className="text-sm text-foreground/80">Email</label>
         <input
@@ -130,7 +159,7 @@ export default function SignupForm() {
         <button
           type="submit"
           formAction={formAction}
-          disabled={!isValid}
+          disabled={!isValid || isPending}
           className="shine-wrap btn-premium h-11 min-w-[7.5rem] text-sm font-semibold hover:cursor-pointer dark:disabled:opacity-70 disabled:cursor-not-allowed disabled:[box-shadow:0_0_0_1px_var(--border)]"
         >
           <span>Sign up</span>
diff --git a/src/utils/supabase/server.ts b/src/utils/supabase/server.ts
index 5314470..fa8cd68 100644
--- a/src/utils/supabase/server.ts
+++ b/src/utils/supabase/server.ts
@@ -1,4 +1,5 @@
 import { createServerClient } from "@supabase/ssr";
+import { createClient as createSupabaseClient } from "@supabase/supabase-js";
 import { cookies } from "next/headers";
 import { Database } from "@/types/db";
 
@@ -30,3 +31,17 @@ export async function createClient() {
     }
   );
 }
+
+export async function createServiceClient() {
+  return createSupabaseClient<Database>(
+    process.env.NEXT_PUBLIC_SUPABASE_URL!,
+    process.env.SUPABASE_SECRET_KEY!,
+    {
+      auth: {
+        persistSession: false,
+        autoRefreshToken: false,
+        detectSessionInUrl: false,
+      }
+    }
+  );
+}