A seat device for a vehicle includes a seat cushion, a slow deformation portion, and a rearward-tilt member. An occupant in the vehicle is to be seated on the seat cushion. The slow deformation portion is disposed in at least a portion of the seat cushion and is configured in such a manner that deformation of the deformation portion proceeds as a result of a load to be applied continuously to the slow deformation portion. The rearward-tilt member is disposed in a load direction in an inner portion of the slow deformation portion. When the slow deformation portion is deformed by the load, the rearward-tilt member is tilted rearward from its position before the load is applied.