Background: Gastric endoscopic submucosal dissection (ESD) has a high rate of complications. However, it is unclear whether BMI affects ESD complications. We aimed to investigate the impact of BMI on ESD complications. Methods: A total of 7,263 patients who underwent gastric ESD were classified into 3 groups according to the Asia-Pacific classification of BMI: normal (BMI <23 kg/m2, n = 2,466), overweight (BMI 23-24.9 kg/m2, n = 2,117), and obese (BMI ≥25 kg/m2, n = 2,680). Adjusted logistic regression analyses were conducted to assess the association between BMI and ESD complications. Results: Compared to the normal group, a lower incidence of perforation and a higher incidence of pneumonia and leukocytosis were found in the overweight and obese groups, and intra-ESD desaturation and hypertension were more frequent in the obese group. After adjustment for confounders, the risk of perforation significantly decreased in the overweight (odds ratio [OR] = 0.24, 95% confidence interval [CI]: 0.17-0.33) and obese (OR = 0.12, 95% CI: 0.08-0.18) groups compared to that in the normal group. Meanwhile, the risk of pneumonia significantly increased in the overweight (OR = 11.04, 95% CI: 6.31-19.31) and obese (OR = 10.71, 95% CI: 6.14-18.66) groups compared to the normal group. During sedation, the obese group had a significantly increased risk of desaturation (OR = 2.81, 95% CI: 1.18-6.69) and hypertension (OR = 1.35, 95% CI: 1.11-1.63) compared to the normal group. Conclusions: High BMI was significantly associated with ESD complications. More caution is needed in cases of obese patients undergoing ESD.