A male thick-billed parrot (Rhynchopsitta pachyrhyncha) was diagnosed with a malignant melanoma of the mandibular beak (gnathotheca). Surgical excision was impossible because of the location of the lesion; a combination of radiation therapy and oral antitumor drugs were used to treat the neoplasm. A whole-body computed tomographic scan showed evidence of metastasis in the lungs; the bird, therefore, was considered to have stage IV disease. Throughout the treatment period, the bird showed no clinical evidence of systemic disease. The bird was given 20 treatments of localized radiation therapy of 2.5 Gray (Gy) for a cumulative dose of 50 Gy. The bird was also treated with piroxicam and cimetidine orally from the time of diagnosis, throughout radiation therapy, and until its death. By the completion of radiation therapy, the initial lesion had decreased considerably in size. The bird survived 2.5 months after radiation therapy was completed but died of complications related to metastatic disease. Necropsy results revealed metastases throughout the body, including lesions in the lungs and liver. To our knowledge, this is the first report of localized radiation therapy and oral antitumor drugs being used to treat malignant melanoma in an avian patient. The radiation therapy did produce tumor response in the form of a reduction in size of the visible tumor.