This paper presents a robust controller for uncertain robot manipulators subject to disturbances which are composed of sinusoids. The controller employs the disturbance observer based controller which can effectively estimate and compensate the effect of plant uncertainties and the disturbances. Assuming that the frequencies of sinusoids are known, we embed the internal model of disturbances into the proposed controller so that the design parameters of the controller can be chosen without using the magnitude of disturbance or its time derivative. A rigorous stability analysis shows that the closed-loop system under the proposed controller behaves like the nominal closed-loop system free of disturbances. Simulation results for a 2-DOF manipulator show the effectiveness of the proposed controller.