Zethus Fabricius is the largest genus within the Eumeninae and is currently divided into four subgenera. Three of them, namely, Z. (Zethoides), Z. (Zethus) and Z. (Zethusculus), are further subdivided into species-groups. The Zethus smithii species-group is among a few that present an elongated stem in the second metasomal tergum, but possess no other distinctive features. Here, a cladistic analysis under implied weighted characters was conducted. The Z. smithii-group was recovered as paraphyletic in relation to Z. hilarianus-group and, thus, both were combined into a single group named after the latter. Furthermore, the species of the Z. smithii species-group (sensu Bohart & Stange) were revised. Zethus dicomboda dicomboda and Z. dicomboda prixii were synonymised. Five new species were described: Z. alessandroi, sp. nov., Z. kaapora, sp. nov., Z. mesocarinatus, sp. nov., Z. pontagrossensis, sp. nov. and Z. scrobalis, sp. nov. A key to the species is provided.