♦ Background: Obesity is increasingly prevalent worldwide, and a greater number of patients initiate renal replacement therapy with a high body mass index (BMI). This study aimed to evaluate the association between BMI and organism-specific peritonitis. ♦ Methods: All adult patients who initiated peritoneal dialysis (PD) in Australia between January 2004 and December 2013 were included. Data were accessed through the Australia and New Zealand Dialysis and Transplant (ANZDATA) Registry. The coprimary outcomes of this study were time to first organism-specific peritonitis episode, specifically gram-positive, gram-negative, culture-negative, and fungal. Secondary outcomes were individual rates of organism-specific peritonitis for the same 4 microbiological categories. ♦ Results: There were 7,381 peritonitis episodes among the 8,343 incident PD patients evaluated. After multivariable adjustment, obese patients (BMI 30 – 34.9 kg/m2) had an increased risk of fungal peritonitis (adjusted hazard ratio [HR] 1.69, 95% confidence interval [CI] 1.18 – 2.42), very obese patients (BMI ≥ 35 kg/ m2) had a significantly higher risk of gram-positive peritonitis (HR 1.15, 95% CI 1.02 – 1.30), while both obese and very obese patients experienced significantly higher risks of gram-negative peritonitis (HR 1.29, 95% CI 1.11 – 1.50 and HR 1.30, 95% CI 1.08 – 1.57, respectively) compared with patients with normal BMI (20 – 24.9 kg/m2). Obesity and severe obesity were independently associated with increased incidence rate ratios of all forms of organism-specific peritonitis with a non-significant trend for severe obesity and gram-negative peritonitis association. ♦ Conclusion: Among Australian patients, obesity and severe obesity are associated with significantly increased rates of gram-positive, gram-negative, fungal, and culture-negative peritonitis.