Isolated cleft of the mitral valve is an uncommon congenital cause of mitral regurgitation. Most of the clefts involve the anterior leaflet and isolated clefts of the posterior leaflet have been anecdotally identified. We report a case of Marfan syndrome in whom an echocardiographic examination revealed a cleft in the posterior mitral leaflet. Marfan syndrome frequently involves the mitral valve causing mitral valve prolapse. Occurrence of a cleft may be an extension of its mitral valve involvement.