The seasonal incidence of acute abdomens, such as appendicitis, is reportedly more common in summer but is reported less frequently in Asia. Additionally, seasonal variations in the severity of acute abdomens have been evaluated insufficiently. This study evaluated the seasonal variations in the incidence and severity of acute abdomens in Japan. This retrospective observational study used a multicenter database containing data from 42 acute hospitals in Japan. We included all patients diagnosed with acute appendicitis, diverticulitis, cholecystitis, and cholangitis between January 2011 and December 2019. Baseline patient data included admission date, sequential organ failure assessment score, presence of sepsis, and disseminated intravascular coagulation. We enrolled 24,708 patients with acute abdomen. Seasonal admissions for all four acute abdominal diseases were the highest in summer [acute appendicitis, (OR = 1.35; 95% CI = 1.28-1.43); diverticulitis, (OR = 1.23; 95% CI = 1.16-1.31; cholecystitis (OR = 1.23; 95% CI = 1.11-1.36); and cholangitis (OR = 1.23; 95% CI = 1.12-1.36)]. The proportion of patients with sepsis and disseminated intravascular coagulation as well as the total SOFA score for each disease, did not differ significantly across seasons. Seasonal variations in disease severity were not observed.