Rhachidelus Boulenger, 1908, is a monotypic snake genus belonging to the tribe Pseudoboini and the family Dipsadidae, distributed only in South America. The genus and the species were described by Boulenger (1908) based on a single male specimen from the state of São Paulo, Brazil, sent at the time by Vital Brazil, a physician and public health officer. In the description, Boulenger was succinct, basing it on data from scalation (number of ventrals, subcaudals, supralabials, infralabials, and postoculars), coloration, biometry, and scale topography. In the present study, based on data collected from 11 new specimens, we propose an emended diagnosis for the species, as well as track its holotype. Rhachidelus brazili can be differentiated from other species of the tribe Pseudoboini by the combination of the following characters: (1) midbody dorsal scales in 25 rows; (2) smooth dorsal scales with two apical pits; (3) enlarged vertebral row; (4) mixed subcaudals (single and divided); (5) single cloacal scale; (6) divided nasal scales; (7) one preocular; (8) two postoculars; (9) two or three preventrals; (10) darkened dorsum; (11) dorsal coloration invading the lateral sides of the ventral scales. In addition, we tracked the holotype of Rhachidelus brazili, since in the description, Boulenger (1908) did not assign a registration number to the specimen, only mentioning that it would be deposited at the British Museum of Natural History, in London. Through contact with the curatorship of the institution's Reptile Collection and through photographs, it was possible to identify the label with the name of Vital Brazil, indicating that it is the only individual sent by him and used by Boulenger to describe the genus and the species. With this, it becomes possible to officially link registration number 1946.1.9.35 to the holotype of Rhachidelus brazili.