A side airbag for installation into a motor vehicle, a vehicle seat with such a side airbag and a motor vehicle with such a vehicle seat are described. The side airbag comprises at least one mounting device for mounting the side airbag on the backrest of the vehicle seat, an outer skin with a first side wall extending forwards, when viewed in the vehicle direction, and a second side wall connected to the first side wall. In order to be able, at as low a cost as possible, to protect the passengers to be protected better against a movement towards the middle of the vehicle, the second side wall extends essentially cross-ways to the vehicle direction when the outer skin is fully deployed and free from external forces, so that at least a third side wall connecting the first two side walls is present.