A four-year-old neutered male nutria (Myocastor coypus) was presented for a one-day history of lethargy and anorexia. A right-sided facial swelling and loose right mandibular fourth molar that exuded caseous exudate from the root were noted; however, the animal continued to decline despite removal of the affected tooth and antibiotic and anti-inflammatory therapy. Radiographs showed a lytic proliferative bony lesion on the right mandible that appeared to expand in size over the course of a week. Due to its declining clinical condition and poor response to therapy, the animal was euthanized. Necropsy revealed an invasive bony neoplasm of the right mandible, histologically consistent with an osteosarcoma, that was invading the mandible and dental arcade, likely contributing to tooth root infection and osteomyelitis. Endocardiosis of the tricuspid valve was incidentally found as well with early cardiac remodeling of the right ventricle. This is the first report of an osteosarcoma and endocardiosis in a nutria.