ABSTRACT Ateralphus Restello, Iannuzzi & Marinoni, 2001 is a Neotropical genus of longhorn beetles composed of nine species. This genus was proposed from splitting Alphus White, 1855 into other two genera: Ateralphus and Exalphus Restello, Iannuzzi & Marinoni, 2001. Even though Ateralphus (nine species), Alphus (four) and Exalphus (18) were recently revised, their validity has not been tested using phylogenetic methods. In this study, we carried out a cladistic analysis of Ateralphus and its related genera, Alphus and Exalphus, based on 44 morphological characters of the adults, to test their monophyly and infer the relationships between their species. Our results support the monophyly of the three genera and recovered two clades that corroborate the species-groups previously recognized in Ateralphus. A new genus, Grandateralphus gen. n., is proposed for one of these clades, which is supported by three synapomorphies: width of upper ocular lobes less than width between the lobe and the coronal suture (character state 6: 0), genae parallel in frontal view (8: 1) and scape gradually expanded toward apex, reaching widest diameter just near apex (9: 2). Grandateralphus gen. n. includes three new combinations: G. lacteus (Galileo & Martins, 2006), comb. n.; G. tumidus (Souza & Monné, 2013), comb. n.; and G. variegatus (Mendes, 1938), comb. n. Notes on the distribution of G. variegatus comb. n. and a new record of E. cicatricornis Schmid, 2014 for Bolivia (Santa Cruz) are provided.