The seeding of endocarditis directly to the non-valvular areas of the heart is extremely rare. We report a case of a 36-year-old woman presenting with sudden-onset lower-extremity pain and swelling, who was found to have mitral valve endocarditis with pulmonary vein vegetation. Echocardiography showed a large linear mitral valve mass coming into contact with the base of the left inferior pulmonary vein with each ventricular contraction. Spread into the atrium from mitral valve endocarditis is generally thought to be caused by jet lesions, whereas direct seeding as in this case is not well described. The location in the pulmonary vein is also unique, with no case reports being found in the literature, although the significance remains to be ascertained.