We describe a new genus and new species of pselaphine rove beetle, Batriscydmaenus tishechkini Parker and Owens, from lowland Panamanian rainforest. The new taxon marks a radical departure from the basic pselaphine anatomical groundplan, with a globose body shape and a dramatic reduction of foveae, sulci and striae—features that are considered plesiomorphic in Pselaphinae. This overt simplification of the integument is typical of myrmecophile and termitophile taxa within Pselaphinae. A probable symbiotic lifestyle of members of Batriscydmaenus is further implied by their compact antennae and the presence of suberect, spatulate setae covering the dorsum. The convergent evolution of this trend towards character loss in inquilinous pselaphines implies a “reductive” ecomorph, specialized for living inside social insect societies in part by abandoning many cuticular features of free-living pselaphines. Batriscydmaenus represents possibly the most extreme manifestation of this ecomorph known to date. Reductive anatomy poses a challenge to taxonomic assignment, but we confirm molecularly that the genus belongs to the tribe Batrisini, using gene fragments amplified from a paratype.