On-off control of hydraulic circuits enables significant improvements in efficiency compared with throttling valve control. A key enabling technology to on-off control is an efficient high-speed on-off valve. This paper documents the design of an on-off hydraulic valve that minimizes input power requirements and increases operating frequency over existing technology by utilizing a continuously rotating valve design. This is accomplished through use of spinning port discs, which divides the flow into pulses, with the relative phase between these discs determining the pulse duration. A mathematical model for determining system efficiency is developed with a focus on the throttling, leakage, compressibility, and viscous friction power losses of the valve. Parameters affecting these losses were optimized to produce the most efficient design under the chosen disc-style architecture. Using these optimum parameter values, a first generation prototype valve was developed and experimental data collected. The experimental valve matched predicted output pressure and flows well, but suffered from larger than expected torque requirements and leakage. In addition, due to motor limitations, the valve was only able to achieve a 64Hz switching frequency versus the designed 100Hz frequency. Future research will focus on improving the prototype valve and improving the analytical model based on the experimental results.