Germline mutations in STK11 (also known as LKB1) are found in individuals with Peutz-Jeghers syndrome (PJS) manifesting with gastrointestinal polyps that contain a prominent stromal component. Epithelia in polyps of Stk11^+/− mice can retain a functional copy of Stk11 (refs. ,), and loss of heterozygosity is not an obligate feature of human polyps, raising the possibility of non-epithelial origins in tumorigenesis. Here we show that either monoallelic or biallelic loss of murine Stk11 limited to Tagln-expressing mesenchymal cells results in premature postnatal death as a result of gastrointestinal polyps indistinguishable from those in PJS. Stk11-deficient mesenchymal cells produced less TGFβ, and defective TGFβ signaling to epithelial cells coincided with epithelial proliferation. We also noted TGFβ signaling defects in polyps of individuals with PJS, suggesting that the identified stromal-derived mechanism of tumor suppression is also relevant in PJS.