A seat assembly for a vehicle includes a seatback and a seat bottom extending away from the seatback. An airbag is supported by the seat bottom and includes a middle member having a first end and a second end. The middle member is spaced from the seatback. The airbag includes a first side member extending from the first end of the middle member toward the seatback, and a second side member extending from the second end of the middle member toward the seatback. When inflated in response to a sensed impact of the vehicle, the middle member, the first side member, and the second side member extend around the occupant. One leg of the occupant is disposed between the middle member and the first side member, and the other leg of the occupant is disposed between the middle member and the second side member.