Background: On June 05, 2017, the U.S. Food and Drug Administration approved the Edwards Lifesciences SAPIEN S3 Valve for transcatheter mitral valve replacement of degenerated bioprosthetic mitral valves. We present a case of the youngest patient (13 years old) to undergo this procedure. Case presentation: At 10 years old, this female patient underwent bioprosthetic mitral valve replacement. Three years later, she began experiencing shortness of breath NYHA class III. A transthoracic echocardiography showed a degenerated bioprosthetic mitral valve and severe mitral stenosis with a mean pressure gradient of 18 mm Hg. After deeming the patient, a high risk for surgery, the heart team opted to perform a transcatheter mitral valve-in-valve replacement. After the valve delivery system crossed the degenerated valve, a SAPIEN valve balloon was slowly inflated without rapid pacing to allow for fine positioning. Transoesophageal echocardiography evaluation revealed a mean gradient of 5 mm Hg. The patient discharged after 2 days. At her 30-month follow-up, she reported regular daily activity with no shortness of breath, and a follow-up transthoracic echocardiography showed normal function of the right ventricle, which also had reduced in size. Her systolic pulmonary artery pressure decreased to 33 mm Hg, the mean pressure gradient of the left ventricular outflow tract was 10 mm Hg, and the mitral valve mean pressure gradient was 6 mm Hg. Case discussion: Transcatheter mitral valve replacement is feasible in young female patients with a small body surface area. Safety and long-term outcomes need more research in this age group.