The invention describes a side airbag (2) of a vehicle occupant restraint system (10) provided to be mounted on a vehicle seat (4) and having at least one fabric cut (6, 7) and one gas cushion (8), wherein the side airbag (2) includes at least three fastening points (12, 14, 16) for mounting on the vehicle seat (4), in particular on a vehicle seat frame, wherein at least a first fastening point (12) and a second fastening point (14) are arranged in the at least one fabric cut (6, 7), wherein the first and second fastening points (12, 14) are arranged, in a condition mounted on the vehicle seat (4), in an upper area of the side airbag (2) and in a vehicle seat transverse direction (Q) are arranged at a distance (a) from each other. The invention also describes a vehicle occupant restraint system (10) comprising a seat-integrated airbag module (30) and a vehicle seat (4) comprising a seat-integrated airbag module (30) each of which comprises a side airbag (2) of this type.