Omothymus is a small genus of large, arboreal tarantulas from Southeast Asia, currently containing four recognized species: O. schioedtei, O. violaceopes, O. fuchsi, and O. rafni. The genus was first established in 1891 by Tamerlan Thorell, based on the Malaysian species Omothymus schioedtei. In 1903, Eugène Simon sank Omothymus into synonymy with Cyriopagopus, and for more than a century these spiders were generally treated under that name.
Modern work on arboreal Asian tarantulas has restored Omothymus as a distinct genus. In 2015, Smith & Jacobi re-examined type material and concluded that Omothymus represents its own lineage within the subfamily Ornithoctoninae, reinstating the genus from synonymy. Further revisions by Gabriel & Sherwood in 2019 refined the diagnosis of Omothymus, added O. rafni, and transferred the famous “Singapore Blue” tarantula to Omothymus violaceopes.
Today, Omothymus species are known from Malaysia, Singapore, Indonesia (Sumatra and nearby islands), and newly reported localities in southern Thailand, where they inhabit humid lowland and hill forests high in tree cavities and crevices.
