Trephionus Bates, 1883, a Japanese endemic genus in the subtribe Synuchina (Coleoptera, Carabidae, Sphodrini), is revised taxonomically based mainly on the shape of the endophallus, a membranous inner sac everted from the aedeagus of the male genitalia. Three known species from central Honshu, T.
kinoshitai Habu, 1954; T.
shibataianus Habu, 1978; and T.
babai Habu, 1978, are re-defined based on this genital character, and five new species are described from the region: T.
cylindriphallus Sasakawa, sp. n., T.
niumontanus Sasakawa, sp. n., T.
inexpectatus Sasakawa & Itô, sp. n., T.
abiba Sasakawa & Itô, sp. n., and T.
bifidilobatus Sasakawa & Itô, sp. n. The observed interspecies differences in endophallus morphology are discussed in terms of the species-level phylogeny and genus-level taxonomy of Trephionus.