A seat apparatus includes a vehicle seat including a cushion member having a bottom portion with a plurality of recessed portions, a plurality of air cells configured to expand by injection of compressed air, and a fluid supply mechanism configured to supply the fluid to the air cells. In the seat apparatus, at least one of the recessed portions is provided at a portion of the bottom portion of the cushion member positioned at a seating portion of the vehicle seat, the hip of a passenger being placed on the seating portion. Each air cell is expandable in a corresponding one of the recessed portions. The fluid supply mechanism is configured to expand at least one air cell by supplying the compressed air thereto and to change the at least one air cell to be expanded by switching a fluid supply destination.