Cor triatriatum sinister (CTS) is a rare congenital cardiac malformation. In CTS, a fibromuscular membrane subdivides the left atrium into 2 chambers. The communication between the 2 chambers is through 1 or more orifices in the dividing membrane. We present an interesting case of a 2-month-old infant with obstructed CTS membrane who first presented on account of poor feeding and failure to thrive. Echocardiography showed a persistent levoatrial cardinal vein (LACV) connecting the left atrium and the innominate vein. This allowed the proximal left atrial chamber to decompress its blood volume into the innominate vein and subsequently the superior vena cava. There was minimal prograde blood flow across the Cor triatriatum membrane, so the majority of pulmonary venous blood ultimately returned to the heart by way of the decompressing vertical vein into the systemic venous circulation. Surgical repair was performed with an uneventful postoperative course. The specific anatomical variant of Cor triatriatum found in our subject has rarely been reported.