A curtain airbag for a vehicle includes a main chamber shaped in such a way that, in the position being mounted on the vehicle, in the inflated state the main chamber covers an inner side structure of the vehicle. The curtain airbag further includes an additional chamber which, in the mounting position, is located on the in driving direction of the vehicle front portion of the main chamber, and deploys in the direction of the interior of the vehicle when the curtain airbag is inflated. An extended section connects the additional chamber and the main chamber at a rim section at the same side and limits movement of the additional chamber during the inflation process.