BACKGROUND: Interleukin-37b (IL-37b), a vital negative regulator of the innate immune system, has been reported to be a tumor inhibitor in different type of cancers. However, little is known about the relationship between IL-37b and hepatocellular carcinoma (HCC). The present study aimed to investigate the potential roles of IL-37b in HCC progression. METHODS: Subjects (n = 237) were recruited, and serum IL-37b was measured using ELISA. The tumor-suppressive capacity and underlying mechanisms of IL-37b in HCC were investigated in vitro and in vivo. RESULTS: Compared to healthy controls, serum IL-37b levels were elevated in chronic hepatitis B (CHB) patients but decreased significantly in HBV-HCC patients, especially for those with portal venous tumor thrombus. Low level serum IL-37b in HBV-HCC patients correlated with high HCC stage and poor overall survival and disease-free survival. In vitro and in vivo, recombinant human IL-37b inhibited proliferation and metastasis in HCC cells. Furthermore, IL-37b inhibited epithelial mesenchymal transition in HCC cells in vitro by downregulating IL-6, pSTAT3 (Y705), N-cadherin, and vimentin expression and by upregulating E-cadherin expression. These effects were partially reversed by transfection of adenovirus encoding human IL-6. CONCLUSIONS: IL-37b inhibits HCC growth, metastasis and epithelial mesenchymal transition by regulating IL-6/STAT3 signaling. Serum IL-37b may be a biomarker for HBV-HCC and its staging.