Penile tumours are rare in dogs. Reported herein is a case of a penile extramedullary plasmacytoma (EMP) in a 5-year-old male cocker spaniel that was brought to a local hospital for an evaluation of a penile mass. The mass was approximately 1.3 cm in diameter at the time of presentation. In fine needle aspiration and histopathological examinations, the neoplastic cells showed eccentric round nuclei, a moderate amount of basophilic cytoplasm, and a peri-nuclear clear zone, consistent with plasma cell morphology. There was nuclear pleomorphism with mononuclear giant cells and occasional bi-nucleation. Round cells on the periphery of the mass demonstrated plasmacytic differentiation. Immunohistochemically, the neoplastic cells stained positive for MUM1 and light lambda chain. Based on the cytological and pathological observations, a diagnosis of penile EMP was established.