Background: Opioid receptors are implicated in cancer progression and long-term patient outcomes. However, the prognostic significance, underlying mechanisms, and therapeutic value of mu-opioid receptor (MOP) in hepatocellular carcinoma (HCC) remain unclear. Methods: MOP expression in human biopsy HCC samples was evaluated using RNA microarrays, quantitative real-time polymerase chain reaction (qRT-PCR), and immunochemical analyses. Molecular and cellular techniques, including siRNA-mediated depletion and lentiviral vector-mediated overexpression, were used to elucidate the functions and mechanisms of MOP. The effect of the MOP agonist morphine in HCC was evaluated both in vitro and in vivo. The therapeutic value of MOP inhibitors in HCC progression and metastasis was investigated with in vitro experiments and subcutaneous and orthotopic HCC mouse models in vivo. Results: Through microarray analysis and qRT-PCR, we identified that MOP is highly expressed in human HCC tumours. High MOP expression in HCC tumours was confirmed by immunocytochemistry and correlated with aggressive clinicopathological features and a worse prognosis. Depletion of MOP suppressed cell proliferation, migration, and invasion, whereas overexpression of MOP promoted cell growth and metastasis in human HCC cell lines. Both clinical and biological evidence revealed that MOP-mediated epithelial-mesenchymal transition promotes HCC metastasis and poor prognosis. Morphine promotes cell proliferation, migration, and invasion in vitro and in vivo in mouse models. More importantly, MOP inhibitors suppressed cell growth, invasion, and metastasis in vitro and in the subcutaneous and orthotopic xenograft models.