Cor triatriatum sinistrum (CTS) is a rare congenital cardiac anomaly characterised by an abnormal septum within the left atrium impairing blood flow to the left ventricle. We report the case of a two-month-old male infant who presented with symptoms of heart failure since the age of two weeks. He was admitted to a local hospital and was managed with antibiotics because of the impression of pneumonia. Due to persistent unresolved tachypnoea and tachycardia, he was referred to Sultan Qaboos University Hospital, Muscat, Oman, in 2019 for cardiac evaluation which confirmed a diagnosis of isolated CTS with severe stenosis and pulmonary hypertension. He underwent an urgent surgical excision of the membrane with uneventful recovery. Keywords: Cor Triatriatum Sinistrum; Congenital Heart Disease; Pulmonary Hypertension; Heart Failure; Case Report; Oman.