This paper deals with the design, improvement, and implementation of a nonlinear control strategy to solve the tra-jectory tracking problem for an autonomous underwater vehicle under model uncertainties and external disturbances. First, a disturbance observer based on high-order sliding mode control is designed to counteract the negative impact of both parametric uncertainties and bounded external disturbances. Then, the nonlinear control is enhanced through injecting the disturbance estimation into the designed controller. The stability of the closed-loop system with the enhanced proposed nonlinear controller is proven by Lyapunov arguments. Finally, real-time experimental results are also provided to demonstrate the effectiveness of the proposed controller.