A restraint system includes a seat defining a cross-seat axis. The seat includes a seat bottom and a seatback supported by the seat bottom. An airbag is mounted to the seat bottom and is inflatable upwardly from the seat bottom to an inflated position. The airbag, in the inflated position, includes a panel spaced from the seatback and two extensions each extending from the panel to the seatback. The extensions are spaced from each other along the cross-seat axis.