Despite the recent realization of Interleukin (IL)-35 in tumorigenesis, its exact impact on colorectal cancer (CRC) progression and prognosis, however, is yet to be elucidated clearly. We thus in the present report conducted comparative analysis of IL-35 levels between CRC patients and matched control subjects. IL-35 is highly expressed in all CRC tissues, which can be detected in vast majority of colorectal cancer cells. IL-35 levels in CRC lysates and serum samples are highly correlated to the severity of malignancy and the clinical stage of tumor. Particularly, a significant reduction for serum IL-35 was noted in patients after surgical resection, indicating that IL-35 promotes CRC progression associated with poor prognosis. Mechanistic study demonstrated a significant correlation between serum IL-35 levels and the number of peripheral regulatory T (Treg) cells in CRC patients, suggesting that IL-35 implicates in CRC pathogenesis probably by inducing Treg cells, while cancer cell-derived IL-35 may also recruit Treg cells into the tumor microenvironment in favor of tumor growth. Together, our data support that IL-35 could be a valuable biomarker for assessing CRC progression and prognosis in clinical settings.