A 72-year-old male patient, who had been on chemotherapy for the treatment of IgG-λ multiple myeloma, presented an enlargement of the testis 3 years and 5 months after the diagnosis. High orchiectomy was then performed, leading to the diagnosis of plasmacytoma. Due to residual disease, treatment with a combination of isatuximab and dexamethasone was initiated. The patient is currently under follow-up without recurrence. While testicular tumors are difficult to diagnose by imaging studies alone and extramedullary plasmacytomas rarely occur in the testis, pathological assessment is critical for treatment planning.