We report the case of an 84-year-old man who developed primary diffuse large B-cell lymphoma of the testes during the course of mycosis fungoides treated with topical medication. He was referred to our hospital due to bilateral testicular masses, and bilateral high orchiectomy was performed. A pathological diagnosis of diffuse large B-cell lymphoma was made after an examination of the surgical specimen. Rituximab-combined miniCHOP chemotherapy with prophylactic intrathecal injection resulted in complete remission without recurrence 1 year after diagnosis. People with mycosis fungoides are known to be at a higher risk of secondary malignancies than healthy individuals; hence, a pathological examination is important to confirm the diagnosis.