The world fauna of the genus Thoron is revised. The genus is described and Neothoron Masner is treated as a junior synonym (new synonymy). Relationships with Elgonia Risbec and other Scelionidae are discussed and a key to world species is presented. Six new species are described: T. dayi, new species (Indonesia), T. dux, new species (Bolivia, Venezuela), T. garciai, new species (Venezuela, Bolivia), T. rex, new species (Bolivia), Thoron rivalis, new species (Brazil, Panama), and T. spinifer, new species (Ecuador). Thoron metallicus Haliday (Holarctic), T. longicornis Masner and Huggert (USA), and T. lautus (Masner), new combination (Ecuador, Bolivia) are redescribed; the female of T. lautus and the male of T. longicornis are described and keyed for the first time. Thoron gibbus Ruthe is treated as a junior synonym of T. metallicus (new synonymy). Species of Thoron are only known to parasitize the eggs of Nepidae (Hemiptera: Heteroptera) with rearings from Nepa Linnaeus and association with Curicta Stål; apparently the genus Ranatra Fabricius is not a host.