Hexasepalum is the priority name for Diodella. In the present paper, we provide a synopsis of Hexasepalum. The genus differs from the remaining genera of the tribe Spermacoceae by the combination of infundibuliform flowers, 2-lobed stigmas, schizocarpic fruits, ventral face of the mericarps with two depressions, seeds with T or Y shaped ventral grooves, and pollen grains with two types of supratectal elements. We propose two combinations, i.e. Hexasepalum domingense and H. vaginale, and the formal synonymization of H. angustifolium (including Diodella crassifolia). We also propose two new synonyms: Diodia rosmarinifolia [= Diodella rosmarinifolia] is placed in the synonymy of H. apiculatum, and Diodia incana is now considered a synonym of H. teres. We choose lectotypes for H. vaginale, and for two synonyms of H. apiculatum. In addition, we describe and illustrate a new species endemic from Bahia, Brazil, H. zappiae sp. nov. Detailed descriptions are provided only for the species imperfectly known; otherwise comprehensive taxonomic descriptions are mentioned for each species. The descriptions of the Hexasepalum species are complemented with a key, geographical distributions, and illustrations of key morphological characters. Line drawings are provided for species not illustrated before in literature.