Examining morphological and anatomical characters, eight bloom-forming Ulva species were identified from La Paz Bay, Gulf of California, Mexico. However, DNA sequencing, using a multi-gene approach ( rbc L, ITS2 and tuf A) revealed that, of the eight species, four species were genetically distinct, none of which belonged to the species they would have been ascribed based on morpho-anatomical observations. The Ulva lactuca and Ulva rigida morphotypes (blades two cells thick, unbranched, attached by a small discoid holdfast) corresponded to Ulva ohnoi ; the Ulva intestinalis morphotype (tubular and smooth thallus, one cell thick) corresponded to Ulva tepida ; the Ulva clathrata and Ulva flexuosa morphotypes (tubular to slightly compressed thallus) corresponded to Ulva torta. The Ulva acanthophora morphotype (more or less tubular, profusely branched) did not correspond molecularly to any currently described species. Attempts to sequence the remaining two species, morpho-anatomically identified as Ulva lobata and Ulva nematoidea , were unsuccessful. This study similarly highlights the high degree of cryptic diversity in the genus Ulva , provides a nomenclature adjustment for several species of the genus Ulva from northwestern Mexico and is the first to report U. ohnoi , U. tepida and U. torta for the region.