evidence: Guyana, located in northern South America north of Brazil between Venezuela and Suriname, includes portions of the Orinoco, Amazon, and Courantyne river basins, a majority of the Essequibo River Basin, and all of the Berbice and Demerara river basins. Partly as a consequence of historical and contemporary drainage complexity, Guyana has a remarkably diverse freshwater fish fauna. Here, we compile the first comprehensive list of Guyana's freshwater fishes, totaling some 657 species-level taxa (97 first records, 117 endemics and at least three introduced species), based on photographic records, published reports, and over 145,000 type and non-type specimens in 29 museum collections. The IUCN conservation status for each species is provided. We separately list an additional 151 undocumented species that are possibly or likely present in Guyana based on their adjacent distributions in either the Orinoco River Delta in Venezuela or the Courantyne River in Suriname. The largest holdings of fish specimen vouchers and frozen fish tissues from Guyana are housed at the Royal Ontario Museum, Toronto (ROM), Auburn University Museum, Auburn (AUM), University of Michigan Museum of Zoology, Ann Arbor (UMMZ), and the Academy of Natural Sciences of Drexel University, Philadelphia (ANSP).