Diverticula of the left atrium are very rare malformations of unknown etiology and may be associated with arrhythmias, thromboembolism, or mitral valve regurgitation. We report a patient with suspected coronary artery disease (CAD) in whom we performed a low-dose computed tomography coronary angiography using prospective electrocardiogram triggering. CAD could be ruled out. Incidentally, we found a diverticulum on the posterior wall of the left atrium combined with an interatrial septal aneurysm. The diagnosis was confirmed by transthoracic echocardiography.