An oscillating weight (I) with variable geometry for a timepiece mechanism has a first and a second part (10; 20), and an axis of rotation (40) shared by the first and the second part (10; 20). At least one part (10; 20) is arranged in order to oscillate about the axis of rotation. A differential mechanism (30) is connected to the first and to the second part (10; 20) so as to vary the position of one part relative to the other through a rotational movement of at least one of the parts about the axis of rotation (40). Owing to the presence of the differential mechanism (30), the user of the watch can vary the geometry of the oscillating weight (I) directly and therefore the position of its centre of gravity, and thus adapt it to their lifestyle (for example, sport mode, normal mode).