Wiener models, which consider a linear dynamical model and a nonlinear output map, can represent a wide range of industrial processes. In this paper, an observer design approach for these systems is proposed. The approach considers a Wiener structure having a nonlinear perturbation term and an implicit nonlinear output map. The observers' gains are obtained by solving an LMI system which considers the one-side Lipschitz constant associated to the nonlinear perturbation and the convex hull associated to the output map derivative. A pH neutralization process illustrates the main features of the design process and the performance obtained with the proposed observer.