The objective of this paper is to develop a robust decentralized observer-based feedback model reference tracking control approach for a class of nonlinear disturbed interconnected systems. The proposed H_{∞} control and observation design method is formulated within an optimization problem involving linear matrix inequalities (LMIs), efficiently solved by a one-step LMI procedure, to compute the decentralized observation and control gain matrices of each subsystem, and to attenuate the external disturbances affecting the subsystems by minimizing a H_{∞} performance criterion. A numerical simulation is highlighted on a power system with three-interconnected machines to demonstrate the effectiveness of the developed control approach despite the interconnections between different generators, nonlinearities in the system, and external disturbances.