BackgroundSubepicardial hematoma due to spontaneous coronary artery rupture (SCAR) is exceedingly rare.Case presentationAn asymptomatic 68-year-old woman with Graves’ disease was introduced to us because of an abnormal right hilar shadow. Graves’ disease had been treated effectively with thiamazole. Without any chest trauma or disease related to coronary artery rupture, computed tomography revealed a 44-mm mass lesion that occupied the front of the right atrium. Two branches arising from the proximal right coronary artery surrounded the lesion and appeared to be the feeding vessels. We suspected a tumor and performed radical surgery. The lesion was completely removed using an ultrasonic scalpel and revealed as organized subepicardial hematoma.ConclusionThis is the rare surgical case of organized subepicardial hematoma due to SCAR. A high grade of suspicion is necessary for the correct diagnosis. Graves’ disease is not considered as an underlying disease of SCAR.