The optimization of electrothermal in-flight anti-icing systems is presented by introducing a general methodology. The optimization goal was to achieve an ice-free area over the protected zone by using the lowest energy possible. The power and/or length of the electric pads are considered as design variables. The optimization procedure is performed via a derivative-free method that typically needs many objective-function evaluations. This would be impractical as aero-icing flow simulation remains computationally intensive when coupled with conjugate heat-transfer calculations, as in the case of ice-protection systems. The cost is even more prohibitive for an optimization process, as a large number of simulations are needed. To make it practical, this work presents a surrogate-based optimization approach using proper orthogonal decomposition, in conjunction with kriging. The results obtained show that the methodology is efficient and reliable in optimizing electrothermal ice-protection systems in particular, and a thermal-based one in general.