© 2017 Society for Mucosal Immunology. The intestinal microbiome in early life influences development of the mucosal immune system and predisposition to certain diseases. Because less is known about the microbiome in the stomach and its relationship to disease, we characterized the microbiota in the stomachs of 86 children and adults and the impact of Helicobacter pylori infection on the bacterial communities. The overall composition of the gastric microbiota in children and adults without H. pylori infection was similar, with minor differences in only low abundance taxa. However, the gastric microbiota in H. pylori-infected children, but not infected adults, differed significantly in the proportions of multiple high abundance taxa compared with their non-infected peers. The stomachs of H. pylori-infected children also harbored more diverse microbiota, smaller abundance of Firmicutes, and larger abundance of non-Helicobacter Proteobacteria and several lower taxonomic groups than stomachs of H. pylori-infected adults. Children with restructured gastric microbiota had higher levels of FOXP3, IL10, and TGFβ expression, consistent with increased T-regulatory cell responses, compared with non-infected children and H. pylori-infected adults. The gastric commensal bacteria in children are altered during H. pylori infection in parallel with more tolerogenic gastric mucosae, potentially contributing to the reduced gastric disease characteristic of H. pylori-infected children.