This paper is concerned with the synthesis of robust guaranteed-cost controllers for networked systems operating under uncertain, bounded, time-varying transmission delays. The uncertainty of the transmission delay results in an uncertain discrete-time networked controlled system (NCS). The main contribution is a synthesis procedure for a static control law which not only robustly stabilizes the system against all admissible time-varying network-induced delays but also addresses performance issues expressed via a quadratic cost function. This delay-dependent procedure (a sufficient condition expressed via LMI) gives rise to a parametrization of the guaranteed-cost controllers allowing thus the investigation for an optimal controller which minimizes the upper bound of the closed-loop cost function. The provided simulation results demonstrate the efficiency of the proposed control scheme.