MicroRNAs (miRNAs) play critical roles in embryonic development and are frequently deregulated in human cancers. The let-7 family members are tumor-suppressing miRNAs and are frequently downregulated in cancer cells. Lin-28 and Lin-28B are RNA-binding proteins highly expressed in embryonic tissues. Lin-28 proteins block let-7 precursors from being processed to mature miRNAs by inducing terminal uridylation and degradation of let-7 precursors. Here, we report that Lin-28B, but not Lin-28, is highly expressed in hepatocellular carcinoma (HCC). Lin-28B expression was more frequently noted in high-grade HCCs with high alpha-fetoprotein levels. Knockdown of Lin-28B by RNA interference in the HCC cell line HCC36 suppressed proliferation in vitro and reduced in vivo tumor growth in NOD/SCID mice. In contrast, overexpression of Lin-28B in the HCC cell line HA22T enhanced tumorigenicity. Overexpression of Lin-28B also induced epithelial-mesenchymal transition in HA22T cells and hence, invasion capacity. Large-scale real-time PCR array analysis revealed that, among 380 miRNAs, only let-7/mir-98 family members were regulated by Lin-28B. Lin-28B overexpression enhanced the expression of the known let-7 targets c-myc and HMGA2. It was also found that Lin-28B enhanced the expression of type 1 insulin-like growth factor receptor in a let-7-dependent manner. These results indicate that Lin-28B regulates tumor formation and invasion in HCC through coordinated repression of the let-7/mir-98 family and induction of multiple oncogenic pathways.