Ruxolitinib is a selective JAK1/2 inhibitor, and its application in treating adult-onset dermatomyositis(DM) has rarely been described. Here, we report a case of an adult-onset DM patient with anti-TIF1γ antibody, who also suffered from essential thrombocytosis related to the mutation. Her clinical symptoms of DM markedly improved after ruxolitinib treatment, whereas her platelet count did not significantly change. In addition, we reviewed previous studies on the treatment of adult DM with ruxolitinib, and summarized reports of the J mutation in adult idiopathic inflammatory myopathy.