Diverticulum of the atrium is a rare and controversial clinical entity, and in the present case a left atrial diverticulum was associated with severe mitral regurgitation (MR). A 68-year-old female with the diagnosis of MR was referred for further cardiac evaluation. Echocardiography revealed severe MR and an accessory cavity behind an enlarged left atrium. She underwent surgical valve replacement, but excision of the diverticulum was not done because its rupture seemed unlikely. A follow-up computed tomography performed 8 months postoperatively demonstrated disappearance of the diverticulum. The etiology of this atrial diverticulum is not clear, but the patient's MR may have played a role in its development.