A 44-year-old woman underwent surgery for an asymptomatic primary tumor of the heart located in the right atrium. The tumor was detected incidentally during follow-up computed tomography for a resected breast cancer. The mass, lying along the lower portion of the right atrial septum, was homogenous and cystic in nature, as detected by transthoracic and transesophageal echocardiography. Complete resection was performed via a median sternotomy under cardiopulmonary bypass. The postoperative course was uneventful. However, the histological result was surprising: the mass was a cardiac lymphangioma.