A wave-energy converter has been studied through the combination of laboratory experiments and numerical simulations. The converter model is a semi-submerged axi-symmetric buoy with a circular cross section with a diameter of 26 cm at the water plane. The buoy is pitching about a fixed external axis oriented such that the buoy works primarily in heave. The laboratory model is equipped with a spring mechanism referred to as WaveSpring, which works to shift the resonance period and increase the response bandwidth of the system. A controlled electric actuator was connected and programmed to provide a velocity-proportional force for power extraction. The buoy mass was varied at two levels and the experimental setup was exposed to a selection of regular and irregular waves. The power take-off (PTO) damping was set as a function of sea state. A mathematical model for global motion response was developed based on linear hydrodynamic theory and rigid-body dynamics. Comparison of laboratory measurements and numerical simulation results shows that the dominant physical effects have been well captured by the mathematical model. Overall, the study gives an experimental verification that a negative spring mechanism mounted in parallel with the power take-off machinery of a wave energy converter may be used to increase the average converted power.