The present invention relates to a device for regulating the assistance power of a bicycle provided with an electric assistance machine, the device comprising:means for detecting the direction of rotation of the pedal assembly of the bicycle; means for measuring the speed of a wheel of the bicycle; means for calculating a total power of the vehicle from the speed information; and means for determining a power set-point of the electric motor from the total calculated power, the direction of rotation of the pedal assembly and an assistance factor chosen by the cyclist. The invention also relates to a method for regulating the assistance power of an electric power-assisted bicycle.