Pseudotriphyllus defoei new species represents the first North American species record of this predominantly Eurasian mycetophagid genus. The species is known from two specimens recovered from a single Berlese sample in the Great Smoky Mountains National Park, Tennessee (type locality, Albright Grove Loop Trail). It differs from other described Pseudotriphyllus by setal pattern, color, development of the pronotal lateral carina, size of the male prosternal fovea, and details of the aedeagus. The genus may be distinguished from all other North American mycetophagid genera by the presence of a median prosternal fovea on males and the width of the last antennal segment, which is narrower than the penultimate antennomere.