The present invention relates to a thorax occupant protection device (10) comprising a side airbag (12), especially for being mounted in a door or the seat of an automotive vehicle, which in the inflated and mounted state of the side airbag (12) at its upper end includes at least one occupant-side inflatable inner wing (22) for shifting the occupant's adjacent arm forward and at least one door-side inflatable outer wing (24) for protecting the thorax, wherein the inner and outer wings (22, 24) in the inflated state are juxtaposed at least in portions and are separated from each other by an indentation (26) of the airbag wall (13) extending from the upper end in the downward direction. The present invention further relates to a method of protecting the thorax of a vehicle occupant in the event of a side impact.