Cor triatriatum is a rare condition. It comprises around 0.1% to 0.4% of all congenital heart malformations. There are two types of this anomaly: cor triatriatum sinister (CTS), which is more common and accounts for 92% of all cases, and cor triatriatum dexter (CTD). This condition usually presents at an early age and is diagnosed mainly during early childhood. In some patients, who have less severe cases of CTS, diagnosis could be made in adulthood. We report an unusual case of a 78-year-old woman who was diagnosed with CTS for the first time. The patient was admitted to the cardiology department with symptoms of chest pain and dyspnoea. Physical examination revealed diffusely diminished breath sounds, with focal wheezing, and irregular heart rate, with no murmurs, while blood pressure was 140/90 mmHg. A transthoracic echocardiogram revealed an accessory membrane in the left atria suggestive of CTS.