We describe an extremely rare case of primary cardiac precursor B lymphoblastic lymphoma (B-LBL) in a 10-year-old boy who presented with nonspecific complaints of fatigue and vomiting for 2 weeks and an episode of syncope. Chest X-ray showed cardiomegaly and echocardiography revealed a large right atrial mass, which was successfully resected. Pathology showed precursor B-LBL arising in the heart and there was no evidence of disease at any extracardiac site. A brief review of the literature is also presented.