A new species of squirrel, Sciurus olsoni, from the late Miocene (Clarendonian) of Nevada, is distinguished from other species of Sciurus by its smaller size and minor differences in the cheek teeth. It is the first Miocene record of the extant genus Sciurus, and the first record of a tree squirrel (tribe Sciurini) in North America between the early Miocene (Arikareean) and the Pleistocene. This late Miocene occurrence in North America suggests that the history of tree squirrels in North America was probably continuous through the Neogene, and the intermittent fossil record is due both to preservation bias and to their never having been abundant as living elements of fossil faunas; they are rare even where they do occur in deposits that are otherwise very richly fossiliferous. The later Tertiary and Pleistocene radiation of tree squirrels may have originated in North America.