A control system for attitude of an underwater unmanned vehicle (UUV) is designed, which compensates for such plant matched uncertainty caused, for instance, by unknown inertia moments along axes of the body-fixed frame and displaced center of gravity. Unlike a great number of published papers, a method of estimation of the uncertainty, which is ideologically close to extended state observers from active disturbance rejection control and robust method by A. Tsykunov, is proposed to solve the problem under consideration. Its main advantage is that only a few hyperparameters are to be chosen by trial and error. First of all, the model of UUV is considered, from which the uncertainty description is expressed. Secondly, the proposed method to design the required control system is described and its stability is rigorously analyzed. The exponential convergence of the augmented tracking error to compact set is ensured. Thirdly, experiments with a real UUV are conducted to verify the effectiveness of the proposed system.