The article presents a case of B cell lymphoma affecting the uterus and spleen in a mare delivered for veterinary examination due to infertility. According to information obtained from the animal’s owner, the mare did not become pregnant despite mating in the last two reproductive seasons. Apart from infertility, the owner did not notice any other problems with the mare’s health. The mare was in good body condition and physical examination revealed no significant abnormalities except scant, serosanguineous vaginal discharge. Rectal palpation of the reproductive tract showed multiple nodular masses located in both uterine horns. Transrectal ultrasonography confirmed the presence of well circumscribed, hyperechoic masses of various sizes (with a diameter of 2 to 6 cm) in the uterine wall protruding into the uterine lumen. Moreover, small amounts of fluid in the uterus were found. Routine hematological and biochemical blood tests showed mild anemia and elevated alkaline phosphatase. Based on these findings, the first diagnosis of uterine neoplasia was made. Therefore, a decision was made to perform a hysterectomy. During laparotomy, in addition to the affected uterus, a very large splenic tumour was found. In view of clinical diagnosis of probable metastatic neoplasia and poor prognosis the mare the owner agreed to euthanize the mare. Post-mortem examination revealed numerous different-sized (with the largest diameter of 6 cm), well circumscribed, firm, gray to white nodular masses expanding the wall of the uterus and protruding into its lumen and a very large irregularly shaped tumour, 30 cm in diameter, in the spleen. On cross-sectional surfaces, masses in both the uterus and the spleen were white with small foci of extravasation. Lymph nodes and other organs showed no evidence of neoplasia. Histological examination of collected masses and immunohistochemical staining showed multicentric diffuse large B-cell lymphoma involving the uterus and spleen.