This "third revision" of M.J. Wynne's "Checklist" is a rigorously prepared and updated compilation of the taxa of benthic marine algae, or seaweeds, currently recognized as occurring in the broad area of the tropical and subtropical Western Atlantic Ocean. Thus, the checklist covers the region from the warm temperate eastern United States to southern Brazil, which is the same domain as the 1960 algal flora of W.R. Taylor. It includes a total of 1,393 species of benthic marine algae: 905 red algae, 175 brown algae, and 313 green algae. When the 185 infraspecific taxa are included, the total tally of current names is 1,578. Taxonomic synonyms are also included and are listed in brackets after the current names. The latest proposals on higher and lower hierarchical levels of systematic relationships are followed, reflecting the often dramatic changes in our concepts brought about by recent molecular-based phylogenetic studies. There are also 563 notes in regard to nomenclature and distribution for many of the taxa treated. This publication includes an extensive bibliography of pertinent literature for the period following the publication of the second revision of the checklist in 2005. The checklist includes a table listing new references by geographic region (country or coastal states of the southeastern USA). This work will be a useful and timely resource to workers on marine algae not only of the Western Atlantic but also on a global perspective because of its synthesis of recent literature and presentation of the most modern classification concepts.