Lymphoma is the most common malignancy among felines and the most common site of this disease is the gastrointestinal (GI) tract. Alimentary lymphoma is characterized by infiltration of neoplastic lymphocytes and mostly affects the lower and upper GI tract, liver, or pancreas. There are different grades of GI lymphoma, called low grade (lymphocytic or small cell), high grade (lymphoblastic, immunoblastic or large cell), and intermediate grade. Although it is common in older cats, clinically vomiting, diarrhea, anorexia and weight loss are observed. In cases with intestinal localization, serum concentrations of cobalamin and folate are decreased, which is thought to be due to possible poor enteric absorption. Although cytology, ultrasonography, direct or indirect x-ray techniques are helpful in diagnosis, histopathological examination of the lymph node in the affected area is required for the main diagnosis. Although an effective treatment method has not been determined in high-grade GI lymphomas with an aggressive course, chemotherapy is mostly the preferred method. Likewise, an effective chemotherapy drug or protocol is not yet known. In this study, we aimed to present the clinical approach, histopathological diagnosis and chemotherapy treatment of an 11-year-old male tabby cat diagnosed with high-grade diffuse alimentary lymphoma. In the clinical examination of the patient with symptoms of anorexia and chronic vomiting, abdominal pain, constipation and lethargy were detected. Hematological examination revealed lymphopenia and anemia. Direct and indirect positive contrast x-rays were taken with the suspicion of obstruction in the GI system. Diagnostic laparotomy was performed in the case, which was cured with treatment in the preoperative period. A biopsy sample was taken from the hyperplastic cranial mesenteric lymph node for histopathological diagnosis and sent to the pathology laboratory. Following the diagnosis of high-grade alimentary lymphoma, a CHOP chemotherapy protocol was created based on the reference literature. In our case, whose general condition and clinical controls were followed up regularly, no relapse occurred after the end of chemotherapy from the date of diagnosis and including the date of case report.