In this paper Agaricus megalosporus sp. nov. is introduced with a description, line drawings, color photographs. Its novelty is supported both by its morphology and phylogenetic analysis. A. megalosporus is characterized by medium to large fruiting bodies, a heavily fibrillose squamulose stipe and relatively large basidiospores. This new species is compared with some morphologically similar species. ITS sequence data analysis by maximum likelihood, unweighted parsimony and Bayesian analysis shows that A. megalosporus should be placed in section Minores.