Cor triatriatum is a rare congenital cardiac malformation characterized by a fibromuscular membrane that divides the left atrium into two distinct chambers. In almost all cases, it is diagnosed in childhood, whereas adult cases are extremely rare. Herein, we describe an unusual case of cor triatriatum in a 55-year-old woman who presented with embolic cerebral infarction. The patient experienced sudden-onset, transient left-sided homonymous hemianopsia and echocardiography and multidetector computed tomography detected a membrane-like structure across the left atrium, confirming the diagnosis of a cor triatriatum. The laboratory examination for hypercoagulopathy was negative. She was conservatively treated with anticoagulation and her neurological manifestation gradually improved.