In this study, a yo-yo vibrational energy harvesting device that can adjust its resonant frequency to the desired excitation frequency using geometrical control is proposed and investigated. The device employs a spring system, suspended proof mass, and set of reels and gears to convert translational input into rotational motion to power its generator. The spring system is composed of two symmetrical oblique springs, and its stiffness and the resonant frequency of the proposed harvester varies with the oblique angle between these springs. The characteristics of the oblique spring system are investigated accordingly, and it is shown that there exists a lower limit of stiffness; therefore, the achievable frequency bandwidth of the harvester can be determined. The method used to design the proposed tunable energy harvester so that it satisfies the desired frequency bandwidth is then provided. A prototype of the device was built and tested on a vibration table, and its performance is illustrated via a comparison with the results of numerical calculations. The results indicate that the proposed device is capable of harnessing the kinetic energy of the ocean waves.