Abstract Context Hyponatremia is the most common electrolyte disturbance in hospitalized patients. Known risk factors are heart or renal failure, excessive drinking, or the use of diuretics. The incidence of hyponatremia may also be influenced by climate. Objective Analyzing the influence of outdoor temperature and relative humidity on the incidence and etiology of hyponatremia. Design Cohort A: cross-sectional study from January 2011 to December 2016. Cohort B: prospective observational study from June 2011 to August 2013. Setting Emergency departments of two tertiary centers. Patients Cohort A: patients with plasma sodium ≤145 mmol/L (n = 222,217). Cohort B: consecutive patients (n = 294) with profound hyponatremia (plasma sodium ≤125 mmol/L). Main Outcome Measure The effects of outdoor temperature and relative humidity on the incidence of mild (sodium 126 to 134 mmol/L) and profound hyponatremia (sodium ≤125 mmol/L) were investigated via logistic regression models. The effects of outdoor temperature and relative humidity on hyponatremia etiology were evaluated. Results In cohort A, 9.9% had mild and 1.08% had profound hyponatremia. Outdoor temperature was significantly associated with the incidence of profound but not mild hyponatremia (P < 0.01, P = 0.3). Relative humidity was not associated with the incidence of hyponatremia. In cohort B, diuretic-induced hyponatremia occurred more frequently with higher outdoor temperatures, whereas other etiologies showed no clear variation with outdoor temperature or relative humidity. Conclusions Higher outdoor temperature, but not relative humidity, seems to be associated with the incidence of profound hyponatremia. Our data suggest that diuretics should be used with caution during hot weather.