BACKGROUND: Cardiac tamponade is a potentially fatal complication of catheter ablation for atrial fibrillation (AF). OBJECTIVES: This study aimed to evaluate the impact of body mass index (BMI) on cardiac tamponade during AF ablation. METHODS: Patients who underwent catheter ablation for AF between April 1, 2016 and March 31, 2018 were analyzed using a Japanese nationwide claims database. Mixed-effects multivariable Poisson regression analysis was performed to investigate the association between BMI and cardiac tamponade. RESULTS: A total of 59,789 hospitalizations (age 65.6 ± 10.4 years, 29% women) with catheter ablation for AF were analyzed. Cardiac tamponade occurred in 647 patients (1.1%). Multivariable analysis revealed that being underweight (BMI <18.5 kg/m2) was associated with an increased risk of cardiac tamponade (relative risk [RR]: 1.42; 95% CI: 1.03-1.95) when compared with having a normal weight (BMI ≥18.5 and <25 kg/m2). Other characteristics that were associated with an increased risk of cardiac tamponade were age ≥75 years, female sex, and a history of heart failure, hypertension, diabetes, and dialysis treatment. CONCLUSIONS: In this analysis of a large nationwide database of patients with AF who underwent ablation, being underweight was independently associated with an increased risk of cardiac tamponade during AF ablation. Clinicians should consider the higher risk of cardiac tamponade in the underweight population and take appropriate measures to reduce this risk.