Pythiosis is caused by the agent Pythium insidiosum, an aquatic oomycete of the kingdom Stramenopila.To describe the symptoms, pathological changes and diagnosis methods of gastric pythiosis in dogs.A three-year-old female German shepherd, with access to wetlands, was attended due to vomiting and recurrent diarrhea of 30 days of duration. A palpable mass in the abdomen filling the left epigastric region was identified in the clinical examination. Simple and contrasted radiological examination and ultrasound of abdominal cavity were performed. The animal was referred for exploratory laparotomy for the removal of the mass. The extent of the mass prevented from the excision and the animal was euthanized. Samples of the tumor mass were collected and sent for morphological study and immunohistochemistry.The changes observed in imaging studies were consistent with gastric pythiosis. In cytology and histopathology, non-septate hyphae were identified, and in immunohistochemistry a strong positivity of anti-Pythium antibodies was observed, confirming the diagnosis of pythiosis.Pythiosis in dogs is diagnosed late and tends to evolve in the animal's death. The definitive diagnosis is by cytology, histology and immunohistochemistry.