In this paper, a robust decentralized excitation control scheme is proposed for multimachine power system transient stability enhancement. First, a direct feedback linearization (DFL) compensator through the excitation loop is designed to eliminate the nonlinearities and interconnections of the multimachine power system. Then, a robust decentralized controller is proposed to guarantee the asymptotic stability of the DFL compensated system considering the effects of plant parametric uncertainties and remaining nonlinear interconnections. The design procedure for an n-machine power system involves in solving n Riccati equations. In the design of the robust nonlinear decentralized controller, only the bounds of generator parameters need to be known, but not the transmission network parameters, system operating points or the fault locations. Since the proposed robust nonlinear decentralized controller can guarantee the stability of the large scale power system within the whole operating region for all admissible parameters, transient stability of the overall system can be greatly enhanced. The design procedure is tested on a three-machine example power system. Simulation results show that the proposed control scheme can greatly enhance the transient stability of the system regardless of the network parameters, operating points and fault locations.