Background Suicide is a significant yet preventable public health issue. Body mass index (BMI) is a readily measurable indicator associated with various health outcomes. However, the relationship between BMI and suicidal death risk is complex and warrants further investigation, particularly within contemporary, non-Western contexts with consideration of potential confounders. The purpose of this study was to investigate the relationship between BMI and the risk of suicidal death. Methods This study was nationwide, retrospective, observational study based on Korean National Health Insurance Service database. We analyzed 4,045,081 participants who were aged > 19 years and underwent national health surveillance in 2009. The participants were categorized according to their BMI (underweight: < 18.5 kg/m², normal weight: 18.5–23 kg/m², overweight: 23–25 kg/m², class I obesity: 25–30 kg/m², and class II obesity: > 30 kg/m²). The primary outcome was the death events caused by suicide which was defined by International…