A bicycle derailleur is basically provided with a base member, a movable member and a connecting structure. The base member includes a bicycle mounting portion. The movable member is movable with respect to the base member between a first position and a second position that is farther than the first position from the base member. The connecting structure movably connects the movable member to the base member. The connecting structure moves the movable member with an actuation ratio that descends and then ascends as the movable member moves from the first position towards the second position.