A neonate was diagnosed with a mediastinal mass after presenting with bradycardia. At surgery, she was found to have a 4-cm mass replacing most of the right atrial free wall. After tumor resection, the right atrium was reconstructed with an extracellular matrix biomaterial that supports native tissue regeneration. Her pathology revealed rhabdomyoma, which is rare in patients without tuberous sclerosis. The procedure was well tolerated but was complicated by narrowing of the superior vena cava that required dilation postoperatively.