A rear derailleur guard for mounting onto a bicycle with an axle, a wheel, a horizontal frame member, a vertical frame member and a rear derailleur. The guard has a main region with an apex, a horizontal member that extends forwardly from the main region and contains a semi-circular horizontal clip, a vertical member that extends upwardly from the main region and contains a semi-circular vertical clip and a lower shield that extends downwardly and forwardly from the main region. When the guard is mounted, the horizontal clip attaches to the horizontal frame member and the vertical clip attaches to the vertical frame member. The axle protrudes between the horizontal member and the vertical member adjacent to the apex. The lower shield has an inside edge, an outside edge and exterior and interior surfaces extending therebetween. The outside edge curves such that it is perpendicular to the interior surface.