A robust decentralized output-feedback guaranteed cost control scheme for a class of uncertain interconnected time-delay systems with a given quadratic cost function is proposed in this paper. The uncertainties are assumed to be norm-bounded. Based on the Lyapunov stability theory, the observer-based guaranteed cost controllers are proposed such that the closed loop system is globally asymptotically stable. The conditions for the existence of the decentralized controller are derived when time-delay is added in the observer. It is shown that the controllers can be constructed in terms of the feasible solution of the linear matrix inequalities (LMI).