An autonomous implantable capsule comprises a capsule body provided with an element for its anchoring to a patient's organ. An electronic unit is powered by an energy harvesting module provided with a pendular unit comprising an inertial mass coupled to an elastic piezoelectric beam forming a mechanical-electrical transducer for converting into electrical energy the oscillations of the beam. A mobile support, integral with the clamped end of the beam and mobile in axial rotation about the axis of the capsule body, can be directed by a controllable driver to adjust the angular position of the support so as to maximize the produced electrical power converted by the mechanical-electrical transducer.