An 8-year-old neutered female Cavalier King Charles spaniel was evaluated for progressing right forelimb lameness. Magnetic resonance imaging revealed that the right-side radial nerves and the caudal brachial plexus were swollen. The histological and molecular biological diagnosis by partial biopsy of the C8 spinal nerve was T-cell lymphoma. Coadministration of lomustine and irradiation was started. However, this therapy was ineffective. At necropsy, neoplastic tissues were seen extending into the subarachnoid space of the spinal cord, liver, pancreas and kidneys as gross findings. A large mass was also identified occupying the caudal thorax. Histologic findings included infiltration in these organs and the mass by neoplastic lymphocytes. To date, involvement of peripheral nerves (neurolymphomatosis) is rarely reported in veterinary species.