Rosai-Dorfman disease (RDD) involves abnormal proliferations of oddly behaving histocytes that are not derived from the Langerhan’s Cell linage. These collections tend to occur within lymph nodes, with occasional extra nodal presentation. While RDD is a rare entity itself, extra nodal cases are even more so, with even fewer reporting cardiac involvement, and previously only in adults. This report describes the disease in a pediatric patient who had the unique feature of an extra nodal cardiac mass. The patient, who was known to have sickle cell disease, was initially erroneously thought to have acute chest syndrome. Sudden changes in the patient’s status, including development of 3rd degree heart block, demanded investigation with additional imaging. Chest CT revealed a mass arising from the cardiac interatrial septum and encircling the entire thoracic aorta. Imaging features of Rosai-Dorfman disease are nonspecific, complicating the diagnosis. We present this case with discussion of this extremely uncommon entity. We describe the diagnostic methods, the differential diagnosis, and the treatment options.