A vehicle seat includes a seatback defining an occupant seating area and an airbag mounted to the seatback. The airbag has a top chamber inflatable above the occupant seating area and elongated from the seatback in a seat-forward direction. The airbag has two side chambers inflatable downwardly from the top chamber on opposite sides of the occupant seating area. The top chamber has a plurality of tubular segments each elongated in the seat-forward direction and disposed in a cross-seat direction relative to each other.