Hypoxia inducible factor-1 (HIF-1) monitors the cellular response to the oxygen levels in solid tumors. Under hypoxia conditions, HIF-1a protein is stabilized and forms a heterodimer with the HIF-1β subunit. The HIF-1 complex activates the transcription of numerous target genes in order to adapt the hypoxic environment in human cancer cells. In gastric cancer patients, HIF-1a activation following extended hypoxia strongly correlates with an aggressive tumor phenotype and a poor prognosis. HIF-1a activation has been also reported to occur via hypoxia-independent mechanisms such as PI3K/AKT/mTOR signaling and ROS production. This article argues for the critical roles of HIF-1a in glucose metabolism, carcinogenesis, angiogenesis, invasion, metastasis, cell survival and chemoresistance, focusing on gastric cancer.