The present disclosure relates to a composition for preventing or treating fibrosis including an inhibitor of cyclin-dependent kinase 17 (CDK17) expression or activity. According to the present disclosure, the inhibitor of CDK17 expression or activity significantly reduces collagen production and cell activity and viability in activated hepatic stellate cells (liver fibrosis cell model), renal tubular epithelial cells (renal fibrosis cell model) in which fibrosis is induced by TGF-β treatment, and alveolar epithelial cells (lung fibrosis cell model) in which fibrosis is induced by TGF-β treatment, indicating that the composition of the present disclosure has an excellent effect in preventing or treating fibrosis.