This article introduces and studies the concept of congruence-injective S-act over monoids S as a new generalization of injectivity in act theory. This approach is based on congruence and extends the notions of quasi-injective and pseudo-injective S-acts. The main outcome of this study is a characterization theorem for congruence-injective S-acts with respect to perfect congruence. From this result, we obtain a new characterization of quasi-injective acts, which parallels in L. Fuchs’s classical result for module theory. These contributions enrich the theory of injectivity in the category of acts.