Background: Obesity is a risk factor for various types of cancer, including gastric cancer. Leptin, an adipocyte-derived hormone, may function in stimulating the proliferation of gastric cancer cell. The purpose of this study was to evaluate the role of leptin in gastric cancer. Methods: Leptin level was assessed by ELISA and was compared in obese patients with gastric cancer (n=23) and non-obese patients with gastric cancer (n=23) as a function of their clinicopathological characteristics. Gastric cancer cell lines were studied to investigate the effects of leptin on the Janus kinase/signal transducer and activator of transcription (JAK-STAT) and MEK signaling pathways using wound healing assay, inhibition study, and ELISA. Cancer-initiating cells derived from gastric cancer cell lines were studied to investigate the effects of leptin to the maintenance of stemness and epithelial-mesenchymal transition using invasion assay, RT- PCR, and immunoblotting. Results: Obesity was associated with elevated leptin level and BMI has positive correlation with the leptin level (p=0.001 for both). Overall survival was not significantly different between the two groups (p=0.098). Leptin induces the migration and invasion of gastric cancer cells by activating AKT and ERK and up-regulating the expression of vascular endothelial growth factor (VEGF). Pharmacological inhibitors of JAK and MEK signaling pathways decreased both leptin-induced migration and invasion and the leptin-induced expression of VEGF. Leptin increased the mRNA and protein levels of the markers for stemness (CD44) and epithelial-mesenchymal-transition (Snail and N-cadherin). Conclusions: Leptin plays a role in gastric cancer by stimulating the migration and invasion of gastric cancer cells via activating the JAK-STAT and MEK pathways and contributes to the maintenance of cancer stemness and metastatic potential. Leptin presents evidence supporting the adverse effect of obesity in gastric cancer. Treatment targeting leptin-associated signaling pathways could be a potential strategy for managing gastric cancer.