Saperda lateralis rileyi, a new subspecies of Cerambycidae from parts of Louisiana, Mississippi, and Arkansas, is described, illustrated, and compared to the nominate subspecies. Intergradation between the two subspecies is described, and a map showing their distribution in the southeastern United States is provided. The synonymy of Saperda lateralis var. disconotataPic, 1907 with Saperda tridentata Olivier, 1795 is confirmed.