Significance: Liver kinase B1 (LKB1) is a serine/threonine kinase often inactivated in human cancer. We demonstrate here that loss of LKB1 expression in cancer cells promotes a progrowth metabolic profile that enables increased cell growth and proliferation. Loss of LKB1 promotes increased tumor cell metabolism through mammalian target of rapamycin complex 1- and reactive oxygen species-dependent increases in hypoxia-inducible factor-1α (HIF-1α). LKB1-null cells are dependent on HIF-1α to maintain cellular ATP and viability under poor nutrient conditions, raising the possibility of targeting HIF-1α for synthetic lethality in LKB1-deficient tumors. Together, our data reveal that regulation of cellular metabolism is a key function of LKB1 that may contribute to its tumor-suppressor function in human cancer.