High-quality data regarding the effect of Helicobacter pylori eradication on the risk of noncardia gastric adenocarcinoma (NCGA) remain limited in the United States. We investigated the incidence of NCGA after H pylori eradication therapy in a large, community-based US population. We performed a retrospective cohort study of Kaiser Permanente Northern California members who underwent testing and/or treatment for H pylori between 1997 and 2015 and were followed through December 31, 2018. The risk of NCGA was evaluated using the Fine-Gray subdistribution hazard model and standardized incidence ratios. Among 716,567 individuals with a history of H pylori testing and/or treatment, the adjusted subdistribution hazard ratios and 95% confidence intervals of NCGA for H pylori-positive/untreated and H pylori-positive/treated individuals were 6.07 (4.20-8.76) and 2.68 (1.86-3.86), respectively, compared with H pylori-negative individuals. When compared directly with H pylori-positive/untreated individuals, subdistribution hazard ratios for NCGA in H pylori-positive/treated were 0.95 (0.47-1.91) at <8 years and 0.37 (0.14-0.97) ≥8 years of follow-up. Compared with the Kaiser Permanente Northern California general population, standardized incidence ratios (95% confidence interval) of NCGA steadily decreased after H pylori treatment: 2.00 (1.79-2.24) ≥1 year, 1.01 (0.85-1.19) ≥4 years, 0.68 (0.54-0.85) ≥7 years, and 0.51 (0.38-0.68) ≥10 years. In a large, diverse, community-based population, H pylori eradication therapy was associated with a significantly reduced incidence of NCGA after 8 years compared with no treatment. The risk among treated individuals became lower than the general population after 7 to 10 years of follow-up. The findings support the potential for substantial gastric cancer prevention in the United States through H pylori eradication.