In this paper, a nonlinear static var compensator (SVC) controller is proposed to improve power system voltage stability. A third order nonlinear dynamical description for the SVC system is developed and used in the controller design. Adaptive and robust control techniques are employed to deal with both constant and time varying uncertainties in a power system with SVCs. The effectiveness of the controller on voltage stability enhancement is demonstrated by a three-bus power system. The simulation results show that the time toward voltage collapse can be avoided which would otherwise happen without the controller. This validates the effectiveness of the proposed nonlinear robust adaptive controller in voltage stability enhancement.