A bicycle-seat power generation device includes a seat, a frame, an electrical generator, an electrical generation module, a mount, and a cushioning unit. The electrical generator includes a spindle gear. The electrical generation module includes a toothed rack and a transmission gear train. The toothed rack has an upper end abutting against an underside of the seat. The transmission gear train has an end engaging the toothed rack and another end engaging the spindle gear of the electrical generator. The mount accommodates the electrical generation module and the electrical generator therein in order to have the electrical generation module and the electrical generator coupled between the seat and the frame. The cushioning unit is also coupled between the seat and the frame and is fixed together with the mount in order to drive the toothed rack of the electrical generation module for effecting power generation. As such, a bicycle-seat power generation device that features simultaneous shock absorption and power generation is provided.