The occurrence of acute ischemic stroke in childhood is rare. The etiology of childhood ischemic stroke is different from that of an adult, and coagulation abnormalities are suspected to be related. Protein S is a vitamin-K-dependent plasma protein that inhibits the coagulation system by serving as a cofactor for activated protein C, and the clinical manifestations of its deficiency are virtually identical to those of protein C deficiency. We reported the case of a child who presented with right side weakness and who was diagnosed as having a cerebral infarction associated with protein S deficiency as the cause of stroke.