Convexocoleus Shin, new genus, in the New World tortoise beetle tribe Mesomphaliini (Chrysomelidae: Cassidinae) is described to accommodate a distinctive new species, Convexocoleus rileyi Shin, from the high mountains of Haiti. The new species exhibits characters typically associated with flightlessness, such as extreme convexity of the body and fused elytral medial margins, traits also characteristic of flightless species in the genus Elytrogona Chevrolat, also in the Mesomphaliini. A phylogenetic analysis of 23 mesomphaliine species, based on 44 morphological characters, revealed a distant relationship between the new species and those of Elytrogona, supporting the conclusion that the suite of traits associated with flightlessness was acquired independently. Convexocoleus rileyi is apparently more closely related to species of Botanochara Dejean, based on shared traits in the shape of the pronotum, elytra, and the basal tooth on the pretarsal claw. Relationships among mesomphaliines are briefly discussed, and modified couplets are provided for the existing key to mesomphaliine genera.