A bicycle base member adjustment assembly is provided for a base member of a rear derailleur. The bicycle base member adjustment assembly basically has a base member stopper plate, a biasing force adjustment member and a biasing force adjusting element. The biasing force adjusting element is operatively coupled between the base member stopper plate and the biasing force adjustment member to selectively set a relative angular position of the biasing force adjustment member relative to the base member stopper plate about a rotational axis of the base member. The base member stopper plate has a stopper that limits movement of the base member of the rear derailleur relative to the base member stopper plate. The biasing force adjustment member engages a biasing element of the base member of the rear derailleur. When the biasing force is adjusted, the position of the base member stopper plate is not changed.