Compound styelid ascidians are distributed in all marine environments and usually exhibit high morphological plasticity and complexity. In particular, Botrylloides, Botryllus, and Symplegma species show high morphological variations leading to confusion in traditional taxonomy assignments and to ambiguity in species delineation. Fewer than 20 species in these genera are registered in the Atlantic region. Here we surveyed the coral reefs of the southern Gulf of Mexico and Mexican Caribbean Sea, barcoded a total of 110 samples collected in seven reefs in 24 sites using mitochondrial cytochrome c oxidase subunit I (or 1), as well as performed a detailed morphological study. Species delimitation analyses of barcoding sequencing revealed twelve botryllid species and three Symplegma species. Two of the botryllid species were identified as Botrylloides niger and Botryllus humilis; the latter is the first record for the Gulf of México. The remaining 10 botryllid species and the two Symplegma species are not currently described in the literature and have no close matches in GenBank. One of the Symplegma samples could not be identified as an existing species and current characters do not support the description of a new species. Here we describe twelve new species, seven in Botryllus: B. bonanzus sp. nov, B. camur sp. nov., B. hartensis sp. nov., B. lambertorum sp. nov., B. nortensis sp. nov., B. tunnelli sp. nov., and B. unamensis sp. nov., three in Botrylloides: B. alacranensis sp. nov., B. ampullarius sp. nov., B. catalitinae sp. nov., and two in Symplegma: S. papillata sp. nov., and S. sisalensis sp. nov. We also present a tabular identification key of Botryllus, Botrylloides and Symplegma Atlantic species.