One example of a nutating fluid-mechanical energy converter includes a stator and a rotor. The stator includes an outer cylinder having a longitudinal passage 101 and an inner guide cylinder disposed longitudinally within the outer cylinder. The inner guide cylinder and outer cylinder are concentric and define an annulus for fluid flow. A longitudinal guide is attached to an outer surface of the inner guide cylinder and extending outwardly toward an inner surface of the outer cylinder. The rotor is positioned in the annulus to be eccentric relative to the outer cylinder. The rotor has a sidewall with a guide opening to receive the longitudinal guide. Fluid flowing through the annulus imparts a torque on the rotor causing the rotor to nutate within the annulus. The nutational motion of the rotor can be converted into a rotational motion using a suitable rotary output device.