A three-year-old neutered male pet rabbit weighing 1.9 kg was presented with a progressively enlarging soft-tissue mass involving the right upper lip and rostral oral region. The rabbit had a previous history of gingival surgery associated with a mandibular incisor fracture. Skull radiographs revealed a large heterogeneous mineralized soft-tissue mass in the right rostral maxillary region accompanied by severe osteolytic destruction of the right maxilla and destructive changes affecting the incisors. The mass was surgically excised from its gingival attachment; intraoperatively, no clinically apparent direct attachment to the maxillary bone was identified. Histopathological examination revealed a highly cellular malignant osteogenic neoplasm composed of oval, elongated, and spindle-shaped neoplastic osteoblasts with marked nuclear pleomorphism and mitotic activity. Irregular islands of eosinophilic, homogeneous, glassy osteoid infiltrated by malignant osteoblasts and bone trabeculae were present. No cartilaginous component was identified. Taken together, the clinicoradiologic and histopathologic findings were most consistent with high-grade surface osteosarcoma. The severe adjacent maxillary osteolysis and incisor destruction supported the aggressive local behavior of the neoplasm, while the apparent lack of gross bony attachment at surgery did not exclude a surface origin. The rabbit recovered uneventfully after excision and showed no clinically evident local recurrence during the documented two-month follow-up; thoracic radiography at that time showed no evidence of pulmonary metastasis. This case highlights the diagnostic challenge of distinguishing a high-grade surface osteosarcoma from an extraskeletal osteosarcoma when the tumor has a large soft-tissue component and extensive secondary bone destruction.