IL-33, a new member of the IL-1F, is widely expressed throughout the body and can be up-regulated by stimulation with proinflammatory factors. It has been identified as a functional ligand for the plasma membrane receptor complex that is a heterodimer consisting of membrane-bound ST2L, which is a member of the IL-1R family, and IL-1RAcP. IL-33 is crucial for the induction of Th2 immune responses. Additionally, under other circumstances, it can also act as an endogenous danger signal. Recently, many studies have demonstrated that IL-33 may be related to the development and progression of fibrotic diseases. It has proinflammatory effects in some fibrotic diseases but has anti-inflammatory effects in others. In this review, the biologic characteristics of IL-33 and the role of the IL-33/ST2 signaling pathway in various fibrotic diseases will be discussed. We hope this overview will provide new insights for the treatment of these diseases.