Abstract: Interleukin-35 (IL-35) produced by regulatory T cells is a newly discovered inhibitory cytokine. It's a heterodimer consisting of IL-27β chain Epstein-Barr virus-induced gene 3 and IL-12α chain p35. In recent years, many studies have found that T cells, B cells, and tumor cells can secrete IL-35, which has a major inhibitory effect on the immune system. IL-35 plays an important role in tumor immune evasion and promotion of tumor progression and metastasis, and is an important factor which promotes the occurrence and development of tumors. Reducing IL-35 secretion may improve disease control, and it is very important to make a thorough inquiry into the role of IL-35 in regulating the activities of tumor cells. This review summarizes the molecular structure, biological functions, and regulatory roles of IL-35 in the development of malignant tumors. It is hoped that it can provide a novel perspective on the judgment of tumor prognosis and tumor targeted molecular therapy in the future.