A center side airbag configured to reduce spacing between an airbag cushion and a passenger in a front passenger seat to safely restrict transverse movement of the passenger in the front passenger seat. The center side airbag includes an airbag cushion configured to be unfolded between a driver seat and a front passenger seat, and a protruding chamber configured to be unfolded by a front passenger seat-side portion of the airbag cushion protruding toward a passenger in the front passenger seat.