Abstract: An S-poset is a generalization of an S-act for a pomonoid S. Let S be a pogroup. In this work, by using S-act theory and partially order theory, injectives in the category of S-posets are studied. It is obtained that for any S-poset A_S, A_S admits an injective hull, which is unique up to isomorphism. Furthermore, the injective hull of A_S is necessarily a minimal injective extension and a maximal essential extension of A, and vice versa.