This paper investigates the control problem of attitude synchronization for an on-orbit servicing spacecraft autonomously docking to a freely tumbling target in space. An optimal sliding surface is designed based on optimal control theory, followed by a robust optimal sliding mode control law designed for attitude synchronization in accordance. Meanwhile, the unknown but bounded external disturbances, parameter variations, model uncertainties, and measurement errors are all considered during the control algorithm design. The salient feature of this control law is that the relative attitude variables will converge to the origin in the sense of optimality with respect to a quadratic cost function once the sliding surface is reached. The simulation results validate the effectiveness and robustness of the designed robust optimal sliding mode control law.