(H)erein, we present a case of 46 years old woman with an extracardiac venous connection between two atria associated with secundum type atrial septal defect (ASD). This interatrial tunnel was not recognized by both transthoracic and transesophageal echocardiographic examinations. Computed tomography suspected a blood flow from right inferior pulmonary vein to inferior caval vein. Consequently, the patient underwent cardiac catheterization. Angiography revealed a tunnel between left and right atrium. To our knowledge, this is the first report of a case with venous tunnel between two atria associated with secundum type ASD in the literature.