A prestressing device extending about an axis between a first plane that is substantially perpendicular to the axis and a second plane that is substantially parallel to the first plane and axially offset relative to the latter. The device includes a first annular body extending in the first plane, and a first series of at least three tabs, each of which is connected to the first body and extending in an essentially radial direction, each of the tabs of the first series including a planar surface extending in the second plane. This arrangement ensures surface contact between the parts of the elements to be subjected to prestressing. The device can be used for example for a rotor, and for example for a rotor of an unducted propeller for a turbine engine.