A seat structure for a vehicle, such as a motorcycle, includes a seat arranged rearwardly of a fuel tank mounted on a vehicle body frame. The seat includes a cushion member, and a bottom plate for holding the cushion member. The cushion member is made of an elastic material. The bottom plate is fixed to the vehicle body frame and to the fuel tank. The cushion member includes a bulging portion which bulges forwardly from a front end portion of the bottom plate and onto an upper surface of a stepped portion of the fuel tank. The bulging portion is supported on the upper surface of the fuel tank.