About 5-10% of human gastric tumors harbor oncogenic mutations in the KRAS pathway, but their presence alone is often insufficient for inducing gastric tumorigenesis, suggesting a requirement for additional mutagenic events or microenvironmental stimuli including inflammation. Assessing the contribution of such events in preclinical mouse models requires Cre-recombinase-mediated conditional gene expression in stem or progenitor cells of normal and transformed gastric epithelium. We therefore constructed a bacterial artificial chromosome containing transgene (Tg) comprising the regulatory elements of the trefoil factor 1 (Tff1) gene and the Tamoxifen-inducible Cre recombinase (CreERT2) coding sequence. The resulting Tg(Tff1-CreERT2) mice were crossed with mice harboring conditional oncogenic mutations in Kras or Braf. Administration of tamoxifen to the resulting adult Tg(Tff1-CreERT2);KrasLSL-G12D/+ and Tg(Tff1-CreERT2);BrafV600E/+ mice resulted in gastric metaplasia, inflammation, and adenoma development characterized by excessive STAT3 activity. To assess the contribution of STAT3 to the spontaneously developing gastric adenomas in gp130F/F mice, which carry a knockin mutation in the Il6 signal transducer (Il6st), we generated Tg(Tff1-CreERT2);Stat3fl/fl;gp130F/F mice that also harbor a conditional Stat3 knock-out allele and found that tamoxifen administration conferred a significant reduction in their tumor burden. Conversely, excessive Kras activity in Tg(Tff1-CreERT2);KrasLSL-G12D/+;gp130F/F mice promoted more extensive gastric inflammation, metaplastic transformation, and tumorigenesis than observed in Tg(Tff1-CreERT2);KrasLSL-G12D/+ mice. Collectively, our findings demonstrate that advanced gastric tumorigenesis requires oncogenic KRAS or BRAF in concert with aberrant STAT3 activation in epithelial precursor cells of the glandular stomach, providing a new conditional model of gastric cancer in which to investigate candidate therapeutic targets and treatment strategies.