Abstract Context Hyponatremia is the most prevalent electrolyte disturbance in hospitalized patients. Previous studies have shown a seasonal variation of profound hyponatremia with higher prevalence during warmer months. Objective This study aimed at analyzing the seasonal prevalence and sex- and age-specific differences of hyponatremia in medical inpatients. Design Nationwide cohort study from January 2009 and December 2015 using prospective administrative data. Setting Medical inpatients. Patients Diagnosis of hypoosmolar hyponatremia. Main outcome measures The primary outcome was the monthly alteration in hyponatremia prevalence. Secondary outcomes were the association of outdoor temperature with hyponatremia prevalence and differences among sex and age groups. Results Of 2 426 722 medical inpatients, 84 210 were diagnosed with hypoosmolar hyponatremia, of whom 61% (n = 51 262) were female. The highest overall prevalence of hyponatremia was observed in July (4.5%, n = 8976); the lowest in December (2.7%, n = 6530). The overall prevalence of hyponatremia in women compared with men was higher by 58% (odds ratio [OR], 1.58; 95% confidence interval [CI], 1.56-1.60). The sex-specific difference was most pronounced in the warmest month of July (mean temperature 20.1°C (OR, 1.76; 95% CI, 1.68-1.84). We observed the strongest association between seasonality and hyponatremia in elderly (>80 years) female inpatients admitted during the month of July (OR, 2.40; 95% CI, 2.20-2.62]). Conclusion The prevalence of diagnosed hypoosmolar hyponatremia in medical inpatients increases during summer months with higher outdoor temperature. Elderly female inpatients were most susceptible to the seasonal rise in hyponatremia prevalence.