Various epidemiologic studies have shown that obesity is associated with hepatocellular carcinoma. Leptin, the key player in the regulation of energy balance and body weight control, also acts as a growth factor on certain organs in both normal and disease states. It is plausible that leptin acts to promote hepatocellular carcinogenesis directly affecting malignant properties of liver cancer cells. However, a direct role for leptin in hepatocellular carcinoma has not been shown. In this study, we analyzed the role of leptin and the mechanism(s) underlying its action in hepatocellular carcinoma cells, which express both short and long isoforms of leptin receptors. Treatment with leptin resulted in increased proliferation of both HepG2 and Huh7 cells and involves activation of signal transducers and activators of transcription 3 (STAT3), AKT, and extracellular signal-regulated kinase (ERK) signaling pathways. Leptin-induced phosphorylation of ERK and AKT was dependent on Janus-activated kinase (JAK)/STAT activation. Intriguingly, we also found that leptin potently induces invasion of hepatocellular carcinoma cells in Matrigel invasion and electric cell-substrate impedance-sensing assays. Leptin-stimulated invasion was effectively blocked by pharmacologic inhibitors of JAK/STAT and, to a lesser extent, by ERK and phosphatidylinositol 3-kinase (PI3K) inhibition. Importantly, leptin also induced the migration of both HepG2 and Huh7 cells on fibronectin matrix. Inhibition of JAK/STAT, ERK, and PI3K activation using pharmacologic inhibitors effectively blocked leptin-induced migration of HepG2 and Huh7 cells. Taken together, these data indicate that leptin promotes hepatocellular carcinoma growth, invasiveness, and migration and implicate the JAK/STAT pathway as a critical mediator of leptin action. Our findings have potential clinical implications for hepatocellular carcinoma progression in obese patients.