BACKGROUND: Peritonitis is one of the major complications of peritoneal dialysis (PD). Although several reports have indicated seasonality of peritonitis, the observation periods were short, and there were no reports from Japan. Therefore, the purpose of this study was to investigate the long-term seasonality of peritonitis in a single institution in Japan. METHODS: Of 126 patients who started PD in our hospital between January 1, 2009, and December 31, 2018, 25 patients (15 men, 10 patients with diabetes) developed peritonitis with a total of 42 episodes. The median age at onset was 63 years, and the median duration from the start of PD to the onset of peritonitis was 22 months. RESULTS: The 10-year incidence of peritonitis was 0.12 episodes per patient-year. Compared with the reference season of winter (December-February), the incidence rate ratios (95% confidence interval) for spring (March-May), summer (June-August), and autumn (September-November) were 1.75 (0.65-4.75), 1.56 (0.57-4.31), and 2.42 (0.94-6.23), respectively. In addition, no seasonality of Gram-positive and Gram-negative organisms was observed. CONCLUSION: No seasonality was evident in the incidence of PD-related peritonitis in our hospital over a 10-year period. These findings suggest that the development of peritonitis in Japanese PD patients is not affected by seasonality.