Agaricus (Basidiomycota) is a genus of saprobic fungi that includes edible cultivated species such as Agaricus bisporus, the button mushroom. There has been considerable ecological, nutritional and medicinal interest in the genus, yet the extent of its diversity remains poorly known, particularly in subtropical and tropical areas. Classification of tropical species has for a large part followed the classification of temperate species. The objective of our study was to examine to what extent this system of classification is appropriate for tropical Agaricus species. Species from temperate sections were therefore compared to the major clades of tropical species using a phylogenetic approach. ITS1 + 2 sequence data from 128 species were used in the phylogenetic analysis. Specimens included four species of genera closely related to Agaricus, 38 temperate species representing the eight classical sections of the genus, and 86 putative species of Agaricus from tropical areas of Africa, Asia and the Americas. Bayesian and maximum likelihood analyses produced relatively congruent trees and almost identical clades. Our data show that (i) only about one-third of tropical species belong to the classical sections based on temperate species; the systematics of the genus therefore needs to be expanded; (ii) among the remaining two-thirds of tropical species, those from the Americas and those from Africa and/or Asia group in distinct clades, suggesting that secondary diversification occurred in these two areas; (iii) in contrast, several clades of classical sections contain American and African + Asian species along with temperate species. In this study, we used approximately 50 distinct species from a small area of northern Thailand, most probably being novel species. This diversity indicates that Agaricus is a species-rich genus in the tropics as well as in temperate regions. The number of species and the hypothetical paleotropical origin of the genus are discussed.