The genus Agaricus currently includes about 500 species in seven subgenera. Continuing an inventory of species in the Caribbean, this manuscript’s objective was to focus on those in Agaricus subg. Spissicaules. This subgenus has been recently proposed and divided into four sections by applying a new taxonomic system in which supraspecific taxa are ranked according to their divergence times. We similarly applied this system using Bayesian evolutionary analyses (BEAST). Phylogenetic analyses were performed using maximum likelihood and Bayesian inference methods, based either on multi-gene sequence data (ITS, ncLSU, and TEF1) or only on ITS, and including new collections from the Caribbean and one from Argentina. The analyses revealed seven major clades, all with divergence time estimated around 24 Ma. Among those, four were known sections of the subgenus, one remains a putative section, and we introduce the remaining two as new sections, A. sect. Fulventes and A. sect. Globoterminales. New samples were distributed among ten species-level clades. For eight of them, new species were proposed. They are distributed as follows: A. nanofulvens and A. basicingulatus are in the two new sections cited above, respectively; A. ciferrianus and A. omphalodiscus are in A. sect. Subrutilescentes; A. baronii, A. furfuripes, A. microincrustatus, and A. parvisporus are in A. sect. Rarolentes. The eight new species and the four sections to which they belong are described in detail. High levels of both species richness and phylogenetic diversity in the Caribbean region contributed to the refinement of the classification of A. subg. Spissicaules.