New records of Douglas's squirrel (Tamiasciurus douglasii) document the species in 2 mountain ranges of the Great Basin where tree squirrels have not been previously reported, including 2 sites in the White Mountains, California, and 1 site in the Desert Creek Mountains, Nevada. In the White Mountains, squirrels were photographed by a camera trap on 9 different dates in winter and spring 2016–2017 at a site on the east side of the range crest near the Crooked Creek Field Station of the White Mountain Research Center (3125 m). In early winter 2018, two Douglas's squirrels were observed and photographed 25 km distant and 716 m lower in Leidy Canyon (2409 m) on the lower east side of the White Mountains, 3.5 km west of the Nevada state line. In the Desert Creek Mountains, Douglas's squirrels were observed on 4 days in autumn 2017 and winter 2018 along Desert Creek from 2005 m to 2307 m. We present evidence to suggest that at least the White Mountains records represent recent colonization(s) and, as such, call into consideration the question of how montane mammals are able to migrate into isolated mountain ranges of the Great Basin during warm Holocene climates.