Abstract Smoking is an established risk factor for gastric cancer development. In this study, we aimed to assess prospectively the association of smoking with gastric cancer risk in 1,446 non-cardia gastric cancer cases and 1,796 controls from China, Japan, and Korea with consideration of Helicobacter pylori infection as a potential effect modifier. Applying logistic regression models stratified by study and adjusted for age and sex we found that current, but not former, smoking was significantly associated with gastric cancer risk [OR = 1.33; 95% confidence interval (CI), 1.07–1.65]. However, the association was significant only in H. pylori sero-positive individuals determined by 3 different sero-markers: overall sero-positivity, sero-positivity to the onco-protein CagA, and sero-positivity to the gastric cancer associated sero-marker HP0305 and HP1564. Specifically, a significant interaction was found when stratifying by HP0305/HP1564 (Pinteraction = 0.01) with a 46% increased risk of gastric cancer among HP0305/HP1564 sero-positive current smokers (95% CI, 1.10–1.93) as opposed to no increased gastric cancer risk among HP0305/HP1564 sero-negative current smokers (OR = 0.93; 95% CI, 0.65–1.33). We confirmed that current smoking is associated with an increased gastric cancer risk, however, only among individuals that are simultaneously sero-positive for the leading causal factor for gastric cancer, H. pylori.