This paper presents an antisway control strategy for underactuated overhead crane systems. Moving the suspended load along a predefined trajectory is not an easy controlling task due to the residual swing at the end of traveling. In this paper, a robust scheme, based on integral sliding mode control and trolley disturbance observer, is derived to guarantee a fast and accurate load transfer in spite of system uncertainties and actuator nonlinearities. In association with the trajectory tracking control law, a generalized twisting algorithm guarantees the practical stability of the load sway error dynamics. A complete analysis of stability of the closed-loop system is done using the Lyapunov theory. Experimental results are presented to show the advantages of the antisway integral sliding mode controller with the trolley disturbance observer by comparing its effectiveness with a conventional approach, based on a PD controller.