Bottlebrush squirreltail (Elymus elymoides [Raf.] Swezey) is an important native bunchgrass for rangeland restoration in western North America. This species is taxonomically complex and has diverged into as many as four subspecies, including subsp. brevifolius, for which four geographically distinct races have been described (A, B, C, and D). Of these four races, only C occurs in the northern Intermountain West. Our objectives were to describe phenotypic and genetic variation within C and to ascertain its taxonomic status. We evaluated 32 populations of C collected across the northern Intermountain West for a battery of biomass, phenological, and functional traits in common-garden settings in the field and greenhouse. Genetic variation was assessed with the use of amplified fragment length polymorphism (AFLP) markers, and correlations were calculated among phenotypic, genetic, environmental, and geographic distance matrices with the use of Mantel tests. Values for these four distance measures were positively correlated, suggesting that environmental heterogeneity and isolation by distance are shaping ecotypic divergence driven by natural selection. We describe three phenotypic zones for C that correspond to previously established ecoregion boundaries. Because genetic data group C apart from subsp. brevifolius races A, B, and D, which originate in the Rocky Mountains and western Great Plains, the so-called race C merits description as a new subspecies apart from subsp. brevifolius.