|Abstract||Akko dionea, a new genus and species of gobiid fish, is described from the shelf waters of Brazil just northeast of the mouth of the Amazon River. It is a member of the Gobiosomini, an endemic American tribe with which it shares two synapomorphies: a 3-221110 dorsal pterygiophore formula and 11 precaudal vertebrae. Within this tribe, its relationships are obscured by a large number of autapomorphic features, some unknown in any other gobiids. The osteology of the head, pectoral and pelvic girdles, and the caudal region are described along with certain aspects of the cranial and girdle myology. The arrangement of sensory papillae is illustrated. The species lacks head pores. Parallelism in structures with other unrelated gobies are believed to reflect adaptations to life on a soft mud bottom. Alepidoeleotris tigris Herre is synonymized with Eleotrica cableae Gingburg, and Cayennia Savage is synonymized with Gobioides Lacepede.