A rear derailleur hanger alignment tool is provided with hanger attachment structure, an elongated main arm, a movable body, an indicator and an indicator positioning structure. The hanger attachment structure is attachable to a rear derailleur hanger. The movable body is movably coupled to the elongated main arm between first and second ends of the elongated main arm to slide axially along a longitudinal axis of the elongated main arm and to rotate about the longitudinal axis of the elongated main arm. The indicator is movably coupled to the movable body to move perpendicular to the longitudinal axis of the elongated main arm. The indicator positioning structure is operatively formed between the movable body and the elongated main arm to selectively maintain the indicator in a predetermined angular position with respect to the longitudinal axis of the elongated main arm.