An 80-year-old, 5.6 kg male intact desert tortoise (Gopherus agassizii) was presented for evaluation of an extensive mass protruding from the ventral margin between the supracaudal scutes of the carapace and anal scutes of the plastron. Computed tomography (CT) scan was performed to evaluate the mass extent, which involved the soft tissues of the caudal body wall and invaded into the nearby caudal carapacial suprapygal and pygal dermal bones. The mass measured ~7.2 cm length x 6.5 cm width x 3.7 cm height on initial CT. An 18G core Temno needle biopsy sample was most consistent with an osteosarcoma. Curative-intent surgical excision involving partial carapacetomy was performed to remove the mass, and histopathology demonstrated a well-differentiated, productive osteosarcoma with narrow surgical margins. Partial wound closure followed by open wound management was performed via use of hydrocolloid and gauze bandages. Progressive wound healing occurred over the subsequent 6 months and bandaging was discontinued approximately 6 months post-operatively. A recheck CT scan was performed at both 6 months and 1 year post-operatively, which revealed smooth ostectomy margins with a small undulant periosteal proliferation consistent with post-surgical remodeling. No overt evidence of metastasis in the form of visceral or pulmonary nodules was appreciated at either follow-up CT scan. This is the first case report of a partial carapacectomy as therapeutic procedure for osteosarcoma and one of the few cases of osteosarcoma in a chelonian in reported literature.