Inverted left atrial appendage (LAA) is a rare surgical complication. Our patient, a boy aged 2 years, was diagnosed with a partial atrioventricular defect with mild regurgitation of the left atrioventricular valve and a large primum atrial septal defect. Direct postoperative transesophageal echocardiography revealed a new left atrial mass attached to the atrial septum, without left ventricle inflow obstruction. Out of concern about the nature of this mass, we chose surgical direct examination. Intraoperatively, we diagnosed it as an inverted LAA accidentally attached to the atrial septum suture line. Awareness of this condition can avoid unnecessary diagnostic and therapeutic procedures.