We report on a postoperative patient with cardiac tamponade caused by posteriorly localized pericardial effusion. In this case, transthoracic echocardiography only suggested mild left atrial compression. However, transesophageal echocardiography demonstrated significant collapse of the left atrial appendage that completely disappeared after pericardial drainage. The left atrial appendage collapse should be a valuable diagnostic sign in this setting. Transesophageal echocardiography would surely be valuable in diagnosing postoperative cardiac tamponade as a result of localized effusion when clinical symptoms and transthoracic echocardiographic findings are atypical.