A parsimony cladistic analysis based on 55 morphological characters was performed for the Cascabela Raf.–Thevetia L. species complex (Apocynaceae), including 22 terminals, sampling at least one species of each genus in Plumerieae sensu Endress and Bruyns. The consensus of the 12 most parsimonious trees (length = 164, consistency index = 0.50, retention index = 0.75) can be correlated to three previous subtribal classifications. The topology supports the monophyly of the Cascabela–Thevetia species complex, but it is not conclusive to whether Cascabela should be recognized as a genus or as a subgenus of Thevetia. Because there are two important and easy-to-diagnose characters supporting each clade (digitiform suprastaminal appendages and embryos not compressed in Cascabela; reniform fruits and segmented endocarp in Thevetia), the recognition of two genera is preferred here. Therefore, the new combination C. pinifolia (Standl. & Steyerm.) Alvarado-Cárdenas & Ochot.-Booth is proposed. Keys and synoptic descriptions for the two genera and their species are provided.