This paper presents a robust nonlinear excitation controller design for synchronous generators in a multimachine power system to enhance the transient stability. The mismatches between the original power system model and formulated mathematical model are considered as uncertainties and modeled through the satisfaction of matching conditions. To design the controller, the partial feedback linearization is used which transforms the original multimachine power system model into several reduced-order linear subsystems and autonomous subsystems. The control law can be obtained for each subsystem and the proposed scheme can be implemented in a decentralized manner provided that the dynamics of the autonomous subsystem are stable. Finally, the performance of the proposed control scheme is evaluated on a 3 machine 11 bus power system following a large disturbance. The results are then compared with those obtained from a partial feedback linearizing controller with no robustness properties.