Mount Washburn, the principal peak in the volcanic Washburn Range, is an important site for both tourism and research in Yellowstone National Park. This paper provides: 1) descriptions of plant community types on Mt. Washburn, 2) biogeographic comparisons of species diversity for several ranges in the North-Central Rockies, and 3) an annotated species list of the alpine vascular flora, including summaries of constancy, local abundance, and preferred habitats. The alpine flora consists of one hundred and twenty-six vascular plant species from seventy-five genera and twenty-eight families. Biogeographic analyses suggest that the flora is depauperate for the region, with relatively low rates of colonization. These results agree with the predictions of the theory of island biogeography for small isolated ecosystems, and emphasize the vulnerability of Washburn to sub-alpine encroachment as the result of climate change.