Human personality traits differ across geographic regions1-5 . However, it remains unclear what generates these geographic personality differences. Since humans constantly experience and react to ambient temperature, we propose that temperature is a critical environmental factor that is associated with individuals’ habitual behavioral patterns and thus with fundamental dimensions of personality. To test the relationship between ambient temperature and personality, we conducted two large-scale studies within two geographically large yet culturally distinct countries: China and the United States. Using data from 59 Chinese cities (N = 5,587), multilevel analyses and machine learning revealed that individuals who grew up in areas with milder temperatures (i.e., closer to 22 °C) scored higher on personality factors related to socialization/stability (agreeableness, conscientiousness, emotional stability) and personal growth/plasticity (extraversion, openness to experience). These relationships between temperature clemency and personality factors were replicated in a larger dataset of 12,499 ZIP-code level locations (the lowest geographic level feasible) within the United States (N = 1,660,638). Taken together, our findings provide a perspective on how and why personalities vary across geographic regions beyond past theories (subsistence theory, selective migration theory, pathogen prevalence theory). As climate change continues across the globe, we may also observe concomitant changes in human personality.