Cor triatriatum sinister is a rare condition caused by a membrane in the left atrium, resulting in left ventricular inflow obstruction. This developmental anomaly is usually diagnosed in childhood. However, a rare presentation during adulthood is observed when the membrane is incomplete. Surgical excision of the membrane is the first line of treatment. We present a 51-year-old woman who underwent successful transcatheter balloon dilation with complete loss of the membrane waist and hemodynamic and symptomatic improvement.