The carabid genus Orthogonius MacLeay is treated, based mainly on materials collected in Thailand through the TIGER project (the Thailand Inventory Group for Entomological Research). Among 290 specimens, 20 species are identified in total, 10 of them are new species: O. taghavianae sp. n. (Nakhon Nayok: Khao Yai National Park), O. coomanioides sp. n. (Phetchabun: Thung Salaeng Luang National Park), O. similaris sp. n. (Phetchabun: Thung Salaeng Luang National Park; Loei: Phu Kradueng National Park), O. setosopalpiger sp. n. (Phetchabun: Thung Salaeng Luang National Park), O. gracililamella sp. n. (Loei: Phu Kradueng National Park; Chaiyaphum: Tat Tone National Park), O. pseudochaudoiri sp. n. (Phetchabum: Thung Salaeng Luang National Park; Nakhon Nayok: Khao Yai National Park), O. constrictus sp. n. (Phetchabum: Thung Salaeng Luang National Park), O. pinophilus sp. n. (Phetchabum: Thung Salaeng Luang National Park), O. vari sp. n. (Cambodia: Siem Reap; Thailand: Ubon Ratchathani: Pha Taem National Park; Phetchabun: Thung Salaeng Luang National Park) and O. variabilis sp. n. (Thailand: Phetchabun: Thung Salaeng Luang National Park; Nakhon Nayok: Khao Yai National Park; Phetchabun: Nam Nao National Park; China: Yunnan). In addition, O. mouhoti Chaudoir, 1871 and O. kirirom Tian & Deuve, 2008 are recorded in Thailand for the first time. In total, 30 species of Orthogonius have been recorded from Thailand, indicating that Thailand holds one of the richest Orthogonius faunas in the world. A provisional key to all Thai species is provided. A majority of Thai Orthogonius species are endemic. Among the ten national parks in which orthogonine beetles were collected, Thung Salaeng Luang holds the richest fauna, including 16 species.