A 7-year-old Chow Pei was referred to an animal hospital for the evaluation of a 1-month history of abdominal distension and exercise intolerance. A systolic murmur was auscultated over the tricuspid valve (TV). Thoracic radiographs showed cardiomegaly, right atrial enlargement, and pulmonary edema. Electrocardiography revealed small QRS complexes and a right-bundle brunch block. Echocardiography also showed right atrial enlargement and tricuspid insufficiency due to the presence of a mass on the posterior leaflet of the TV. The mass was excised by intracardiac surgery and histological evaluation revealed myxosarcoma. This is a case report of a diagnostic and therapeutic approach to myxosarcoma of the TV in a dog.