Cytokine-mediated immunity plays a crucial role in the pathogenesis of various autoimmune diseases, including rheumatoid arthritis (RA). Recently, the IL-1-family-related cytokine, IL-33, was detected at high levels in experimental inflammatory arthritis and in the early phase of human RA, and was reported to exert profound pro-inflammatory effects in several experimental autoimmune models. Moreover, administration of IL-33 leads to the development of severe inflammatory arthritis, suggesting that IL-33 may be therapeutically relevant in RA, and the targeting of IL-33 or the IL-33 receptor has been proposed as a potential therapeutic approach for autoimmune diseases such as RA. In this article, we discuss the biological features of IL-33 and summarize recent advances in our understanding of the role of IL-33 in the pathogenesis and treatment of RA. It is hoped that this information may aid the development of novel therapeutic strategies for RA.