Juvenile dermatomyositis is a rare systemic vasculopathy that may sometimes present with acute complications. We report here the case of a 7-year-old boy with severe dermatomyositis associated with thrombocytopenia and blurry vision. The presence of schistocytosis and the secondary occurrence of hemolytic anemia were consistent with a diagnosis of thrombotic thrombocytopenic purpura (TTP). Further investigations demonstrated the association of TTP with muscular microangiopathy and Purtscher-like retinopathy. Retinal and hematologic involvements dramatically improved after the initiation of plasma exchange in emergency. This report emphasizes that early recognition of TTP and prompt plasmapheresis are important in a child with severe juvenile dermatomyositis associated with thrombocytopenia.