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Globally, increasing air moisture can limit not only the human body’s ability to cool down, but also that of the Earth’s surface. This interaction, however, remains understudied—especially across subtropical-to-tropical regions that experience high levels of humidity. Here, we examine how air moisture influences nighttime land surface temperatures (LSTs), as well as what this interaction means for thermal comfort overnight. During summer months across seasonally muggy Southern Florida, during which time air moisture reaches the highest values of any place in the United States, we examine: 1) how air moisture, measured as specific humidity, varies with urbanicity and geography; 2) whether air moisture limits LST cooling at night; and 3) the implications of increased air moisture and nighttime LSTs for thermal comfort overnight. Based on data from 30 weather stations, we find that nighttime LST remains higher and closer to daytime highs in areas with increased specific humidity, highlighting a decreased surface cooling phenomenon in more humid areas. Both urban and coastal areas exhibit higher nighttime LSTs that are strongly related to thermal discomfort—also resulting in more time spent at or above critical thresholds for thermal discomfort overnight. This analysis therefore underscores the importance of heat mitigation and adaptation strategies that reduce LSTs and anthropogenic heat inputs in seasonally muggy climates in order to increase thermal comfort, especially near coastlines at lower latitudes.