A side airbag for motor vehicles is provided. The side airbag comprises a gas bag which is to be arranged as intended on a vehicle seat, so that, in order to protect a vehicle occupant situated on the vehicle seat, it can be deployed laterally beside the vehicle occupant, and a gas generator for filling the gas bag with a gas, so that the same deploys. The gas bag and the gas generator are configured and provided to be arranged on the vehicle seat such that in the deployed state the gas bag supports itself on a backrest frame of the vehicle seat.