S. gallolyticus is one of the pathogenic agents of endocarditis, and mitral valve aneurysm is a rare but potentially devastating complication. We present a case of S. gallolyticus aortic valve endocarditis with concomitant anterior mitral valve leaflet aneurysm. Patient underwent surgery before aneurysm perforation, and postoperative course was uneventful. Time of surgery is crucial to avoid severe complications due to aneurysm rupture.