Human hepatocellular carcinoma (HCC) is a highly invasive tumor with frequent distant metastasis, which is the main cause for the poor prognosis. However, the mechanisms for metastasis remain poorly investigated. MicroRNAs (miRNAs) have been implicated in HCC progression. MicroRNA-122 (miR-122) is considered as a tumor suppressor in human cancer. In the present study, miR-122 expression was found to be significantly lower in HCC than the level in normal tumor-adjacent tissues. miR-122 was clearly silenced or downregulated in five HCC cell lines (HepG2, Hep3B, MHCC97H, Huh7 and SMMC-7721) compared with normal hepatocytes (LO2). HCC patients with low expression of miR-122 had a poor 3-year survival. Univariate analysis and multivariate Cox regression analysis indicated that miR-122 is an independent prognostic factor in HCC. Downregulation of miR-122 promoted proliferation and inhibited apoptosis in Hep3B cells. We found that the public miRNA database (TargetScan) predicted that PKM2 may be a target for miR-122, and the 3'-untranslated region (3'-UTR) of PKM2 contains a highly conserved binding site for miR-122. To identify this, pre-miR-122/anti-miR-122 were respectively transfected into the Hep3B cell line. We found that miR-122 overexpression significantly reduced the level of PKM2. Moreover, knockdown of PKM2 significantly increased miR-122 inhibitor-mediated Hep3B cell apoptosis and reduced miR-122 inhibitor-mediated Hep3B cell migration and invasion. Moreover, re-expression of PKM2 partially abrogated miR-122-induced HCC cell growth arrest and apoptosis in vivo. In conclusion, miR-122 serves as a prognostic biomarker and induces apoptosis and growth arrest by downregulating PKM2 in HCC.