A seat support structure for a working vehicle comprises a fixed plate, a parallel link mechanism, a supporting plate, a rotation shaft, a rotation plate, a seat, a rear upper shaft, and a rotation lock unit. The fixed plate is fixed on a vehicle body of the vehicle. The parallel link mechanism includes a front link and a rear link. The supporting plate is connected to the fixed plate via the parallel link mechanism. The rotation shaft is fixed on a center portion of the supporting plate. The rotation plate is rotatably supported on the rotation shaft. The seat is attached to the rotation plate. The rear upper shaft connects the rear link to the supporting plate. The rotation lock unit is provided on the rear upper shaft.