A derailleur including an articulated quadrilateral mechanism, having four parallel articulation axes is provided. At a first articulation axis, between a fixed member and a first connecting rod, the fixed member includes a first forked structure embracing the first connecting rod. The first forked structure includes two opposite flanges having respective holes aligned along the first articulation axis, and the first connecting rod includes a hole aligned with the first articulation axis. A pin is inserted through the two holes of the first forked structure and the first connecting rod. The use of the fixed member creates a more favorable distribution of the stresses induced in the first connecting rod and the pin by the traction of the derailleur control cable.