Abstract: Members of the Galaxauraceae are all calcified and have a wide distribution from tropical to temperate regions. Of the four genera of Galaxauraceae, only Dichotomaria contains two distinct thallus forms, which are (1) a flattened form with either narrow (e.g. D. marginata) or wide (e.g. D. diesingiana) branches, and (2) a terete form with either slender (e.g. D. papillata) or robust (e.g. D. obtusata) branches. In this study, we present a molecular phylogeny of new Dichotomaria samples from the Indo-Pacific region and describe a new species, D. hommersandii S.L. Liu & S.M. Lin, from South Africa, based on rbcL sequence analysis and morphological evidence. Dichotomaria hommersandii is characterized by a thallus consisting of terete smooth branches (0.5–1.5 mm in diameter), a moniliform branching pattern with constrictions at the nodes, ramification of young branches derived from the damaged terminal regions of the old branches, and cystocarps containing a conspicuous fusion cell. Our phylogenetic analyses indicate that the terete thallus morphology could be the ancestral character state in Dichotomaria.