A 6-yr-old male Vancouver Island marmot (Marmota vancouverensis) presented for poor hibernation, weight loss, and symmetric trunk alopecia. An abdominal interstitial cell tumor was identified and surgically removed. Serum levels of estrogen were markedly elevated before surgery and decreased after tumor removal, indicating that the tumor had been functionally secretory. Nine months later, the marmot presented with respiratory stridor. A large boney nasal mass was identified radiographically and evaluated by computed tomography (CT) prior to surgical debulking. The marmot did not recover from anesthesia. Pathologic findings included a nasal osteosarcoma with lysis of the cribriform plate, and endocardial fibrosis with degenerative changes within the adjoining myocardium. This is the first known report of nasal osteosarcoma and interstitial tumor in a Vancouver Island marmot.