Dilated cardiomyopathy (DCM) is one of the leading causes of heart failure in children with diverse clinical characteristics. To date, DCM with a giant atrium as the first manifestation is rare and has not been reported in previous literature. We report a case of a male infant born with a significantly enlarged right atrium. Due to worsened clinical symptoms and the risk of arrhythmias and thrombosis, we performed the surgical reduction of the right atrium. Unfortunately, DCM and a progressive re-enlargement of the right atrium appeared during midterm follow-up. The mother's echocardiogram also suggested DCM, and the patient was eventually considered for a diagnosis of familial DCM. This case may expand the clinical spectrum of DCM and reminds us of the importance of good follow-up of children with idiopathic dilatation of the right atrium.