This paper presents an integral sliding disturbance observer (I-SDOB) to compensate for unknown disturbances for a class of nonlinear systems. To guarantee the existence of a sliding mode for disturbance estimation, the proposed I-SDOB needs only a small switching gain compared with conventional sliding disturbance observers, which leads to further alleviation of chatter. Moreover, the stability analysis of the controller-observer system is given based on the Lyapunov theory. Applications of the proposed scheme to a two-link robotic manipulator have been conducted, and experimental results confirm the effectiveness of the proposed scheme.