In captivity, male yellow-nosed cotton rats grow to a maximum head and body length of about 155 mm (6 in) with females being slightly shorter and heavier.

The fur on the head, back, and sides is a dull gray color, apart from the snout and a ring round the eye which are ochre, features that distinguish this species from other members of the genus Sigmodon.

Its habitat is mountainous areas with grassy meadows, rocky slopes, shrubland, ponderosa pine, Douglas fir, and pinyon-juniper woodland.

It shares its habitat with the fulvous harvest mouse (Reithrodontomys fulvescens) and southern pocket gopher (Thomomys umbrinus) and uses the burrows of the latter, as well as cavities under boulders.

In these habitats, it makes runs between the clumps of grass, but in sparsely vegetated areas, these runways are difficult to discern.

It feeds primarily on blue grama grass (Bouteloua gracilis), but also the green parts of other plants such as muhly, three-leaf groundsel, gumweed, and three-awn tangle-head, and less frequently on seeds and fruits.