Concurrent seminoma and malignant lymphoma of the testis is rare. We present a case of concurrent seminoma and peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS) in a 54-year-old man who complained of painless left testicular enlargement. Radical left orchiectomy was performed. Macroscopically, the tumor (4.0 × 3.0 cm) was creamy, soft, and homogeneous, and microscopic evaluation revealed an alveolar structure of large cells that formed sheets, as well as colonization by other abnormal cells in a 1.0 × 1.0 cm area. The portion of the tumor comprising large abnormal cells was diagnosed as a seminoma, which was positive for c-kit by immunohistochemistry; the other portion was diagnosed as CD3/CD8, TIA, and granzyme B-positive PTCL-NOS. These two portions were clearly differentiated from one another. The patient received CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisolone) therapy and achieved complete response for 50 months. To our knowledge, this is the first reported case of synchronous advanced seminoma and PTCL.