A 9-year-old castrated male Shih-tzu dog was referred to us because of chronic vomiting. The patient’s hematological, radiographic, ultrasonographic, endoscopic, and histological examinations were evaluated for diagnosis. Hematologic analysis indicated moderate anemia and azotemia. Based on imaging study, oval-shape mass was identified in gastric pylorus area. A proliferative mass was found on endoscopic examination and we performed biopsy using grasping forceps. The histopathological findings of the biopsy specimens indicated hypertrophic gastritis and Y-U pyloroplasty was performed. However, the histopathological examination of surgically resected mass was identified as tubular adenocarcinoma of stomach. Then carboplatin chemotherapy was performed 4 times for 13 weeks. The clinical sign such as vomiting were resolved gradually after surgery and chemotherapy, and the patient’s condition was managed favorably until recently (30 months after surgery). This case report demonstrated clinical features, imaging studies, endoscopic characteristics, histopathological and immunohistochemical features of gastric tubular adenocarcinoma as early gastric cancer in a dog.