A twelve-year-old male castrated domestic shorthair cat was presented for rapidly progressing respiratory distress. The cat was depressed, tachypneic and moderately responsive. Ultrasonography showed a mediastinal mass associated with a significant pleural effusion that needed tapping every five to seven days. Ultrasound guided biopsy yielded a diagnosis of clear cell thymoma upon histopathology. After complete staging procedures, the owner elected to treat the cat with electrochemotherapy (ECT) using systemic bleomycin. Two sessions of ultrasound guided ECT were performed at two week intervals with trains of biphasic electric pulses applied using needle electrodes until complete coverage of the area was achieved. The treatment was well tolerated and resulted in partial remission (PR). Additional sessions were performed on a monthly basis. The cat is still in PR after fourteen months. ECT resulted in improved local control and should be considered among the available adjuvant treatments in pets carrying visceral tumors.