Adults and various immature stages of the stag beetle Torynognathus chrysomelinus Bomans, 1986 were found inside the nests of Pseudolasius ants in Peninsular Malaysia. This paper provides a detailed description of T. chrysomelinus, which is the first ever record of myrmecophily in this genus and the third record in the family Lucanidae. This is also the first record of myrmecophilous Coleoptera with Pseudolasius ants as a host. The description includes the illustration of the male genitalia, mouthparts, larva, and pupa of this genus for the first time. Furthermore, the systematic position of T. chrysomelinus was inferred by the molecular phylogenetic analysis of the family Lucanidae. The results suggest that Torynognathus is a sister taxon to the genus Aegus.