An 11-year-old male castrated Australian Shepherd was presented with a history of lethargy, panting, and weight loss for 1 month. Physical examination revealed a moderately enlarged spleen. Laboratory abnormalities included thrombocytopenia and marked hypercalcemia, with hyperglobulinemia, hypoalbuminemia, and a monoclonal spike in the beta-globulin region on serum protein electrophoresis. Serum total calcium concentration was markedly increased (16.5 mg/dL, reference interval 8.9-11.4 mg/dL) but ionized calcium concentration (1.39 mmol/L) was within the reference interval (1.25-1.45 mmol/L). Isosthenuria was noted, but the dog was not polyuric or polydipsic. Serum parathyroid hormone concentration was within reference limits and parathyroid hormone-related peptide concentration was 0 pmol/L. Radiographic findings were largely unremarkable. Results of cytologic evaluation of a fine-needle aspirate specimen from the spleen indicated plasma cell neoplasia. Based on the results of the electrophoresis, splenic aspirates, radiographs, and hypercalcemia, a diagnosis of splenic multiple myeloma was made. The marked hypercalcemia, normal ionized calcium and parathyroid hormone concentrations, and lack of osteolytic lesions indicated a presumptive increase in protein-bound serum calcium, likely due to binding to molecules of the paraprotein (M protein). Protein binding of calcium in dogs with multiple myeloma should be considered as a potential mechanism of elevated total serum calcium concentration.