The impacts of summer heat extremes are mediated by the moisture content of the atmosphere. Increases in temperatures due to human-caused climate change are generally expected to increase specific humidity; however, it remains unclear how humidity extremes may change, especially in climatologically dry regions. Here, using in situ measurements and reanalyses, we show that specific humidity on low humidity days in the American Southwest has decreased over the past seven decades, and that the greatest decreases co-occur with the hottest temperatures. Hot, dry summers have anomalously low evapotranspiration that is linked to low summer soil moisture. The recent decrease in summer soil moisture is explained by declines in pre-summer soil moisture, whereas the interannual variability is controlled by summer precipitation. Climate models project continued declines in pre-summer soil moisture but increases in summer precipitation, leading to uncertainty as to how summer soil moisture and hot, dry days will change in the future.