A 36-year-old man was referred to our hospital because of heart failure, and severe mitral regurgitation due to restriction of the posterior mitral leaflet was detected by an echocardiogram. We performed posterior leaflet extension with a patch of bovine pericardium. The postoperative course was uneventful, and the patient was discharged on postoperative day 23. Posterior leaflet extension is useful for mitral regurgitation due to posterior leaflet restriction.