Background: This study was performed to investigate the association between body mass index (BMI) and gastric cancer in East and Southeast Asia where most of gastric cancer is non-cardia gastric cancer. Methods:Onthe basis of 8,997 gastric cancer cases among theAsia Cohort Consortium participants from China, Japan, Korea, and Singapore (N = 538,835), we assessed gastric cancer risk according to BMI by calculating hazard ratios (HR) and 95% confidence intervals (CI) using the Cox proportional hazard regression model. Results: A U-shaped associations between BMI and gastric cancer risk were observed. Gastric cancer risks in underweight group (<18.5 kg/m2) and in obesity group (≥27.5 kg/m2) were higher than reference BMI group (23-24.9 kg/m2; HR, 1.15; 95% CI, 1.05-1.25 for underweight; HR, 1.12; 95% CI, 1.03-1.22 for obesity, respectively). The associations of underweight and obesity with gastric cancer risk were consistent in the analyses for non-cardia gastric cancer, intestinal-type gastric cancer, and late-onset gastric cancer. No significant association of underweight and obesity with the risk of cardia gastric cancer, diffuse-type gastric cancer, and early-onset gastric cancer was observed. In addition, we found that the U-shaped association between BMI and gastric cancer risk remained in nonsmokers, while only underweight was related to increased gastric cancer risk in smokers. Conclusions: BMI has a U-shaped association with gastric cancer risk in East and Southeast Asian population, especially for the non-cardia gastric cancer, intestinal-type gastric cancer, and late-onset gastric cancer. Impact: Future studies with consideration of anatomic location and histology of gastric cancer are needed to establish the association of underweight as well as obesity with gastric cancer risk.