A novel point absorber wave energy converter with a tuned inertial mass (TIM), which is capable of significantly increasing the energy absorption and broadening the effective bandwidth, is proposed in this paper. The mechanism of the TIM has originally been introduced in the field of civil engineering as a passive energy absorber for structures subjected to external loadings such as earthquakes. It relies on attaching an additional tuning spring and a rotational inertial mass to the primary system, to improve the energy absorption performance by amplifying the displacement of the damper. Thus, considering typical point absorbers modeled as a mass-spring-dashpot system similar way to civil structures, the application of the TIM to wave energy converters can be expected to have a significant effect. In this paper, numerical investigation on the power generation performance of a point absorber with the TIM is conducted under random sea waves. The amplitude response and power generation performance are compared with the conventional point absorber, considering both non-resonant and resonant buoy cases. It is shown that by properly designing the tuning spring stiffness and generator damping, the rotation of the generator can be amplified compared to the buoy, increasing the power absorption drastically.