Armillaria altimontana, previously considered North American biological species (NABS) X, is described as new. To date, it appears that A. altimontana prefers higher-elevation, mesic sites within the dry, conifer forest zone of western interior North America. This species has been found on hardwoods and conifers and is associated most commonly with Abies-dominated forest types in southern British Columbia, Washington, Oregon, Idaho and northern California. Partial elongation factor 1-alpha (tef1) sequences were generated from six isolates of A. altimontana originating from three locations in northern Idaho. Phylogenetic analyses of all 10 North American Armillaria species were carried out with maximum parsimony and maximum likelihood. Results indicate that isolates of A. altimontana formed a monophyletic group and clustered with A. calvescens, A. cepistipes, A. gallica and A. nabsnona, which is in agreement with recent phylogenetic studies of Armillaria.