Since the initial western Atlantic collections in the Florida Keys and Bermuda during the mid-1800s, Helminthocladia calvadosii sensu auct. (type locality: Calvados, France) has also been identified from the Caribbean Sea and as far south as northern Brazil. Prior to this study, collections from the eastern and western Atlantic had not been compared using molecular-assisted alpha taxonomy. Recent winter-spring collections of H. calvadosii from Bermuda display an overall habit that is distinct from eastern Atlantic plants of the same species, appearing more similar to H. reyesii (type locality: Canary Islands). Utilizing markers for the mitochondrial COI-5P, we have elucidated the relationships between Bermudian isolates and H. calvadosii from near the type locality, verifying their generic placement within the Liagoraceae and demonstrating their distinctiveness. Using vegetative and reproductive characteristics, we conclude that specimens historically identified as H. calvadosii from Bermuda represent a novel species, and propose Helminthocladia kempii Popolizio, C.W. Schneid. et Chengsupanimit sp. nov. for them.