Herein, we report a rare case of TTF-1-positive primary gastric adenocarcinoma that was initially misdiagnosed and treated as postoperative gastric metastasis from primary lung cancer.<h4>Case presentation</h4>A 58-year-old man underwent thoracoscopic right upper lobectomy with systemic lymph node dissection for lung cancer of the right upper lobe. This evidence concerns the gene TTF1 and gastric adenocarcinoma.