This paper proposes an adaptive integral sliding mode control design (AISMC) for a variable speed wind turbine (WT). Its main objective is to capture maximum power when wind speeds are above the rated value (region III), while reducing fatigue due to overloads. The design implements an adaptation mechanism for the upper bounds of the norm of the uncertainties to smooth the pitching action and reduce chattering. System stability was established using the Lyapunov theory. The effectiveness of the proposed approach was validated on a 5 MW variable speed WT modeled using the fatigue, aerodynamic, structures, and turbulence (FAST) aeroelastic simulator. A comparison to a standard SMC showed a considerable decrease in fluctuations of generator torque and generator speed, and subsequently a reduction in mechanical stress when wind speeds are above the rated value. It also confirmed the AISMC’s ability to reduce chattering.