Leiomyosarcoma was diagnosed in the heart of a 3-year-old female guinea pig. Neoplastic tissue was located in the ventricular septum extending into the right ventricular lumen, but was not obvious grossly. Microscopically, the mass was nodular and infiltrative and was composed of streams and bundles of spindle cells. Immunohistochemically, the neoplastic cells expressed vimentin and α-smooth muscle actin. There was no cross striation of the muscle cells. This is the first report of cardiac leiomyosarcoma in a guinea pig.