Cor triatriatum dexter is a rare congenital heart malformation in which a persistent right sinus venosus valve divides the right atrium into two chambers. Before echocardiography, this anomaly has been rarely diagnosed before surgery or death. This is a case of cor triatriatum dexter in an adult with lifelong exertional cyanosis and dyspnea. A definitive diagnosis of cor triatriatum dexter with associated heart defects was best made by transesophageal echocardiography at 47 years of age. Subsequent surgical intervention confirmed all of the echocardiographic findings and successful correction of the defects was performed.