A male Major Mitchell's cockatoo (Lophochroa leadbeateri) of unknown age presented with an ulcerated mass on the ventral tail caudodorsal to the cloaca. An impression smear of the mass showed spindle cell atypia. Multiple biopsies were submitted for histopathology with inconclusive results. A CT scan revealed a soft tissue mass causing compression of the cloacal lumen. The patient underwent surgical debulking, and a core of the mass was submitted again for histopathology, which reported it as fibrosarcoma. Under repeated general isoflurane gas anesthesia, the patient received a course of definitive radiation therapy totaling 60 Gy and divided in 3 Gy X 20 fractions. By treatment completion, the lesion had decreased in size with necrotic debris on the surface. Surrounding tissues appeared healthy and no adverse effects were observed. As of 1.5 years post-treatment, the mass appears completely healed with no signs of reoccurrence. This case suggests that radiation therapy with this protocol could be an effective treatment option for fibrosarcoma in avian species.