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The short-tailed weasel (Mustela erminea) is a Holarctic boreo-montane species that has one of the largest natural geographic ranges of any mammal. At its southern range limits in arid western North America, the species is rare, and factors associated with its distribution are poorly understood. We hypothesized that the availability of voles (Microtus) as prey was a key limiting factor for the distribution of the species. Consequently, we predicted that important variables associated with the occurrence of short-tailed weasels included snow cover and riparian habitat, both of which are linked to abundant vole populations in this region. We tested these predictions in New Mexico using ecological niche models developed with a maximum entropy algorithm (MaxEnt) and based on bioclimatic and biophysical variables. Consistent with our predictions, distribution of the short-tailed weasel was primarily determined by high winter snowfall and proximity to perennial streams. Descriptive habitat data associated with specimen records supported a close association of short-tailed weasels with riparian zones, especially those characterized by the presence of sedges and ponds made by the American beaver (Castor canadensis). We conclude that the short-tailed weasel is facing conservation threats in this region due to a fragmented distribution, degradation of riparian habitats, altered small mammal prey communities, and climate change. We recommend that the short-tailed weasel be considered in conservation planning in this region and that management focus on means to enhance riparian habitat and curtailing dangers posed by climate warming.