The North American monsoon (NAM) is an important source of precipitation across the southwestern United States (US). The approximate northern boundary of this feature crosses the Navajo Nation, in the Four Corners region, where NAM rains have long been important to the livelihoods of Native Americans. Relatively little is known about the characteristics and hydrological significance of the NAM in this region. Here we report a new 4-year record of stable H and O isotope ratios in monsoon-season rainfall and water resources across the Navajo Nation. Monthly precipitation samples collected at 39 sites document a characteristic pattern of ²H- and ¹⁸O-enrichment associated with monsoonal precipitation. These changes are weakly correlated with local precipitation intensity, however, and the correlation that does exist is dominated by sub-cloud evaporation effects. In contrast to precipitation amount, monsoon-season isotopic values exhibited limited spatial variability across the region, and after correction for sub-cloud evaporation Navajo Nation values were similar to those from a site in southern Arizona. Airmass back-trajectory analysis suggests that the uniformly high NAM isotope values across the region may reflect 1) a region-wide shift from mid-latitude to low-latitude moisture sources at the onset of the peak monsoon, and 2) substantial land-surface recycling of NAM moisture in upwind regions. Comparison of precipitation isotope data with surface and groundwater values implies that, despite its hydroclimatic significance, monsoon rainfall contributes little to rand subsurface water resources. This highlights the monsoon’s importance for warm-season land-surface ecology and hydrology critical to residents of the Four Corners region.