Abstract: A mechanical reducer for a turbomachine for an aircraft, this reducer having a sun gear with internal splines and an external toothing, a ring gear which extends around the sun gear and which has an internal toothing, planet gears which are arranged between the sun gear and the ring gear and which each have at least one external toothing meshing with the external toothing of the sun gear and the internal toothing of the ring gear, and a planet carrier which carries first bearings for guiding the planet gears in rotation. The reducer can further include at least one second bearing for guiding the sun gear in rotation. The at least one second bearing can be mounted between the sun gear and the planet carrier.