Information on neoplasms in animals has increased over time, and these studies have helped in the management of reptiles that present tumors. There are similar incidences of neoplasms between homeothermic and ectothermic animals. Furthermore, there are usually more than one type of tumor present. The treatment of wild animals afflicted with cancer usually happens late, contributing to their low life expectancy. Thus, the present work aimed to describe an infrequent case of oral tumor in Boa constrictor. The tumor was observed in an adult female animal, raised in an exhibition area of the Zoo and Botanical Park of the Emílio Goeldi Museum, located in Belém, State of Pará, Brazil. Macroscopically, the mass presented morphologically with an irregular, multilobulated surface, color that varied from white to grey, hemorrhagic areas and its extension was 3.9 x 2.3 cm. The neoplasm was surgically removed, and the histopathological evaluation revealed an adenocarcinoma, with a papillary-like development pattern and a moderate degree of differentiation. The animal died three months after diagnosis due to starvation. The necropsy showed that there was tumor recrudescence and no metastases. Given the impossibility of surgical removal with a greater margin of safety, and adjuvant therapies, this condition favoured the resurgence of the neoplasm. This compromised the animal’s ability to feed and consequently lead to death. Malignant neoplasms in reptiles may have an unfavourable clinical evolution for the maintenance of life, requiring specific therapeutic care such as chemotherapy. Scientific contributions on tumors in these animals are essential for the medical treatment of wild animals, and the conservation of wild species.