Mooreonuphis bidentata, a new species of onuphid polychaete, is newly described from the Mexican Caribbean. It was found in dead coral rocks at depths of 2.2 m and can be distinguished from its congeners by having simple filaments branchiae from chaetigers 19–24, as well as bi- and tridentate pseudocompound falcigers and large median simple tridentate hooded hooks in the first four chaetigers. Including M. bidentata sp. nov., the genus Mooreonuphis includes 19 species, which have been exclusively collected on the American coasts, mainly in tropical and temperate waters. From the Caribbean Sea, six species have been recorded. Of these, M. bidentata sp. nov, M. cirrata, M. dangrigae and M. intermedia are the only species having their locus typicus in this tropical region. A taxonomic key is presented for all the species of Mooreonuphis recorded from both sides of the American seas.