The present invention relates to an electric assist device for a bicycle, comprising an electric machine (4) provided with a support arm (7) and comprising a rotor shaft (16) on which a roller (5) for driving by means of a free wheel is supported, the support arm being mounted on the bicycle by means of a pivot shaft (8) for pivoting between a drive position and a spaced-apart position, positioning means (28) which can be actuated for releasably holding the support arm in a spaced-apart position, means (44) for controlling the operation of the electric machine that are capable of supplying power to the electric machine in order to produce a motor torque in the direction in which the wheel of the bicycle is driven when moving forward, and that are capable of outputting an electrical pulse to the electric machine in order to produce a reaction torque in a direction opposite that of said motor torque, causing the support arm to pivot and actuating said positioning means.