Mitral valve regurgitation is one of the most common heart valve diseases and mitral valve repair is the favored therapy, in which a part of the mitral valve is resected. To improve preoperative planning of this challenging surgery, patient-specific mitral valve replicas have been developed on which the repair can be simulated. However, there is no possibility yet to transfer the planning from the replica to the surgery of the patient. To solve this problem, we developed a patient-specific instrument with a cutting template, intraoperatively visualizing the part of the mitral valve to be resected as planned on the replica. To realize this instrument, the surgeon first simulates mitral valve repair by resection on a patient-specific mitral valve replica. This postoperative mitral valve replica is then digitalized and from it and a preoperative mitral valve model the instrument with cutting template is automatically designed and then 3D printed. An expert heart surgeon successfully tested the functional principle of the instrument on a pig mitral valve.