Here, based on phylogenetic analyses of 18 taxa and 57 morphological characters, we propose a new firefly genus, Costalampys gen. nov., to accommodate eleven species. Five new species are herein described: C. bella sp. nov., C. capixaba sp. nov., C delicata sp. nov. (designated as type species), C. joanae sp. nov. and C. minima sp. nov. In addition, six species are redescribed and transferred from other genera: C. bisbinotata (Pic) comb. nov., transferred from Platylampis Motschulsky; C. decorata (Olivier) comb. nov., transferred from Ethra Laporte; C. pauper (Olivier) comb. nov., transferred from Cladodes Solier; as well as C. klugii (Motschulsky) comb. nov., C. quadriguttata (Gorham) comb. nov., and C. tricolor (Gorham) comb. nov., transferred from Lucidota Laporte. Costalampys gen. nov. is tentatively placed in Lampyrinae, and is diagnosed by: antennae with 11 articles, III–X basally flabellate, lacking dense and upright bristles; clypeus connected to frons by membrane, pygidium rounded; sternum VIII mucronate; phallus with dorsal plate enlarged apically, projecting ventrally and partially embracing the internal sac. Our phylogenetic analyses supported both the monophyly of Costalampys gen. nov. and the new combinations proposed. However, the relationship among congeneric species was poorly resolved. Finally, we provide illustrations of diagnostic features, distribution maps, as well as a key to Costalampys gen. nov. species, based on males.