Fire is an important driver of ecological processes in semiarid environments; however, the role of fire in desert grasslands of the Southwestern US is controversial and the regional fire distribution is largely unknown. We characterized the spatial distribution of fire in the desert grassland region of the southwestern United States, which includes a large portion of the Upper Gila River watershed. Our objective was to investigate the influence of soil properties and ecological site groups compared to other commonly used biophysical variables for modeling fire probability. Soil-landscape properties significantly influenced the spatial distribution of fire ignitions. Bottomland ecological sites (i.e., soil-landscape classes) experienced more fires than expected in contrast to those with coarse soil textures and high rock fragment content that experienced fewer fire ignitions than expected. Influences of mean annual precipitation, distance to road/rail, soil available water holding capacity (AWHC) and topographic variables varied between ecoregions and political jurisdictions and by fire season. AWHC explained more variability of fire ignitions in the Madrean Archipelago compared to the Chihuahuan Desert. Understanding the spatiotemporal distribution of recent fires in desert grasslands is needed to manage fire and predict responses to changing climate.