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An epigeous fungus forming sequestrate basidiomata with reduced lamellae is described as new to science. A phylogeny based on ITS, LSU, and ef-1 alpha markers demonstrate that P. secotioides nests with strong support in the Psathyrellaceae, more precisely in the Psathyrella candolleana clade, representing the first secotioid form known in the Psathyrellaceae. Psathyrella secotioides is sister to a clade encompassing P. candolleana, P. badiophylla, P. leucotephra, and P. hymenocephala. Dark brown spores without a germ pore and a hymeniform pileipellis structure are synapomorphic characters shared with the rest of the species of the P. candolleana clade (Psathyrella sect. Spintrigerae). On the contrary, longer spores with thicker walls, and the apparent loss of cheilocystidia are derived characters, probably linked with producing secotioid basidiomata and related to the loss of forcible spore discharge. Estimation of divergence times using a molecular clock suggests that Psathyrel la secotioides diverged from the core P. candolleana group around 3.6-5.1 Mya (1.6-9.5), approximately when the climate aridization that originated the Sonoran desert started. Psathyrella secotioides is described macro-and microscopically in detail, and microphotographs under optical microscope and SEM are provided.