Introduction: Lymphoma is the most represented hematopoietic tumor in dogs and more commonly treated with chemotherapy. STAT3 is an oncogenic protein involved in the regulation of cellular mechanisms. STAT3 has been observed to be constitutively activated in many kinds of human malignancies including hematopoietic tumours, such as lymphoma, leukemia, and multiple myeloma. The aims of the present study were to evaluate the expression of STAT3 in spontaneous canine lymphoma. Methods: Twenty lymphomas of different histotype were included in the study. Formalin fixed tissue samples were processed for standard histopathological diagnosis and for immunohistochemical evaluation for CD3, CD79, STAT3 and STAT3-p-tyr705. Statistical analysis was performed to evaluate significant relationships between STAT3 expression vs tumour grade and phenotype. Results: The dogs included in the study were 11 males, 3 castrated males, 3 females and 3 spayed females; mean age was 9.26 + 2.84 years, ranging from 4.0 to 14.0 years. Eleven on 20 cases (55.0%) and 9/20 cases (45.0%) were classified as high grade and low grade lymphomas, respectively. B cell lymphomas represented 70.0% (14/20) of the cases included in the study, and the remaining 30.0% (6/20) were classified as T lymphoma. Thirteen on 20 cases (65.0%) were STAT3 positive, and 9/20 (45.0%) were STAT3-tyr-705 positive. Discussion: To our best knowledge this is the first preliminary study reporting STAT3 and STAT3-tyr-705 expression in canine lymphoma. We observed that STAT3 expression higher in high grade than in low grade lymphoma.