Spontaneous regression of an arteriovenous malformation (AVM) is a rare condition observed in 0.3%-1.3% of patients with AVMs and is most likely caused by hemorrhagic events. The regression of an unruptured AVM is rarer than that of a ruptured AVM. Moreover, due to its low frequency of occurrence, the etiology and natural course of spontaneous regression of an AVM is still unclear. This is the first report presenting a case of a spontaneous regression of an unruptured AVM caused by a gradual drainer vein thrombosis that was suspected to result from hypercoagulability due to protein S deficiency.