This fifth revision of M. J. Wynne's "Checklist" is an exhaustively prepared and updated compilation of the taxa of benthic marine algae, or seaweeds, currently recognized in the broad area of the tropical and subtropical Western Atlantic. Thus, this checklist covers the region from the warm temperate eastern United States (Cape Hatteras of North Carolina). Bermuda, the Caribbean, the Gulf of Mexico, eastern Central America, Colombia, Venezuela, to southern Brazil, which corresponds to the same domain as the 1960 algal flora published by W. R. Taylor. This fifth revision of the checklist includes a total of 1,707 species of benthic marine algae: 230 members of the Ochrophyta (Pelagophyceae Phaeophyceae, and Xanthophyceae), 1,131 Rhodophyceae, and 346 Chlorophyceae. When 156 infraspecific taxa are included, the total tally of current names is 1,863. Taxonomic synonyms are also included and are listed in brackets after the current names. There have been many molecular-based phylogenetic studies in recent years, and their impact on classification has been incorporated into the present system in the various groups. There are 565 notes in regard to specific points related to nomenclatural issues, new records, new taxa, and other pertinent information. This publication includes an extensive bibliography of pertinent literature primarily for the period following the publication of the fourth revision of the checklist in 2017. This 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 its presentation of the most modern concepts of algal classification.