A 51-year-old male was unexpectedly diagnosed with unroofed coronary sinus atrial septal defect (CSASD) by coronary computed tomography angiography for a complaint of epigastric pain. As there was no persistent left superior vena cava (LSVC), we planned to undertake a totally endoscopic MICS approach. A 4-cm skin incision was made on the right lateral chest wall under general anesthesia. The 4th intercostal space was used to enter the chest and the right femoral vessels were cannulated for cardiopulmonary bypass. After cross-clamping of the aorta, a right-sided atriotomy incision was made on the left atrium. The CSASD was located at the ventral and caudal regions of the mitral valve and the defect was closed using a bovine pericardial patch. No major postoperative complications were observed. The patient was discharged 7 days after the operation as postoperative transthoracic echocardiography revealed no residual shunt. We thus observed that totally endoscopic MICS patch closure via left atriotomy using a right-side approach for CSASD without LSVC was beneficial to the patient.