The Oriental helotrephid genus Idiotrephes Lundblad, 1933, is taxonomically revised. Species discrimination is based on male genitalia and female terminalia. Three species groups are recognized. The I. chinai group contains I. chinai Lundblad, 1933 (type species; from Sumatra, Borneo, and West Malaysia) and three newly described species; I. asiaticus sp. n. (from Vietnam, Thailand, and west Malaysia); I. yupae sp. n., and I. polhemusi sp. n. (both from Thailand). The I. maior group contains I. maior Papáček, 1994; I. meszarosi Papáček, 1995 (both from Vietnam), and I. hainanensis sp. n. (from Hainan, China). The I. thai group consists of two newly described species from north and northeast Thailand, I. thai sp. n. and I. shepardi sp. n. In addition, some features of biology and morphology of the ovipositor are also included.