Sigmodon arizonae Mearns, 1890, is a cricetid commonly called the Arizona cotton rat. S. arizonae is a large species of cotton rat with brownish black dorsal pelage and silvery or whitish underparts. Ranging from central Arizona in the United States south to Nayarit in Mexico with a disjunct population along the lower Colorado River in southwestern Arizona and California, it is primarily associated with riparian corridors, but also is found in more arid habitats such as semidesert grassland. Two subspecies of S. arizonae appear to be extinct and a 3rd subspecies is imperiled. However, in the majority of its range, S. arizonae is considered common and is not of conservation concern.