A 43-year-old woman with exertional dyspnea and a history of surgically repaired atrial septal defect was referred for a transthoracic echocardiogram, which demonstrated a large, mobile mass in her right atrium. The mass was further characterized with contrast transesophageal echocardiography (TEE) and delayed enhancement MRI, which together suggested a thrombus, attached to the eustachian valve and prolapsing through the tricuspid valve. The mass was resected and the diagnosis confirmed on histopathologic examination. This case illustrates the utility of a multimodal approach in characterizing cardiac masses.