An uncoupling pulley includes a wheel rim including a first area, intended for receiving a belt connecting the wheel rim to a first power-transmission element, and a second area extending from the first area; a hub rigidly connected to a second power-transmission element; a ring gear including a first portion located under the second area and a second portion presented in the shape of a resilient cylindrical skirt extending, from the first portion, along the longitudinal axis, the cylindrical skirt of the ring gear including a plurality of longitudinal slots and consequently a plurality of portions separated from each other by one of the slots, the ring gear being capable of rotating relative to the wheel rim; and a device for driving the ring gear; and a resiliently deformable element, one end of which is attached to the hub and another end of which is attached to the ring gear.