A 9-year-old male Chihuahua was referred to our hospital for lymphocytosis. There was a marked increase in small to middle-sized lymphocytes in the peripheral blood smear and cytology of the spleen. Clonal analysis of lymphocytes using peripheral blood showed clonal expansion of B cells. Bone marrow examination confirmed an increase in small lymphocytes that accounted for more than 50% of all nucleated bone marrow cells. Chronic lymphocytic leukemia was suspected based on the above results; however, histopathological evaluation of the spleen revealed marginal zone lymphoma.