The H _∞ roll control system for the Hydrofoil Catamaran is presented in this paper. The roll dynamics of the ship changes as the speed and other state variables change, and the ship is always under the disturbance from the waves and winds. Taking these two aspects of the control into account, the roll conrol system is defined as the mixed sensitivity problem of the H _∞ robust control theory. The system was designed and validated by the actual sea testing using the prototype ship “Exceller.” The robustness and stability to the disturbance from the wave were clearly observed in the actual sea testing.