Caeoma cassiopae sp. nov. (Uredinales) is described on the arctic-alpine shrub Cassiope tetragona. It was found in three locations in the Rocky Mountains of west-central Alberta, and is the first rust reported on the genus Cassiope. The sori resemble the uredinia of species in the genus Chrysomyxa. The host in the Ericaceae also suggests affinity with that genus. However, the spore morphology, studied by light and scanning electron microscopy, does not resemble known species of Chrysomyxa.