A seat device for a vehicle includes a seat cushion, a slow deformation portion, and tilted standing members. An occupant in the vehicle is to be seated on the seat cushion. The deformation portion is disposed in at least a portion of the seat cushion and configured in such a manner that the deformation portion is deformed as a result of a load to be applied continuously to the deformation portion. The standing members stand in a load direction in an inner portion of the deformation portion and configured to be tilted as a result of the deformation of the deformation portion. When the outer and inner portions of the deformation portion are deformed by the load, the standing members in the outer and inner portions and the standing members in the inner portion are tilted respectively forward and rearward, from their respective positions before the load is applied.