Metabolic adaptability is essential for tumor progression and includes cooperation between cancer cells with different metabolic phenotypes. Optimal glucose supply to glycolytic cancer cells occurs when oxidative cancer cells use lactate preferentially to glucose. However, using lactate instead of glucose mimics glucose deprivation, and glucose starvation induces autophagy. We report that lactate sustains autophagy in cancer. In cancer cells preferentially to normal cells, lactate dehydrogenase B (LDHB), catalyzing the conversion of lactate and NAD+ to pyruvate, NADH and H+, controls lysosomal acidification, vesicle maturation, and intracellular proteolysis. LDHB activity is necessary for basal autophagy and cancer cell proliferation not only in oxidative cancer cells but also in glycolytic cancer cells. Display omitted •Lactate supports lysosomal acidification and autophagy in cancer•Lactate oxidation by LDHB yields protons that fuel lysosomal V-ATPase•Targeting LDHB selectively blocks autophagy in oxidative and glycolytic cancer cells•Targeting LDHB is a promising anticancer approach Brisson et al. show that lactate dehydrogenase B (LDHB) is critical for lysosomal activity and autophagy in cancer cells. Silencing LDHB selectively inhibits the proliferation of both oxidative and glycolytic cancer cells over normal cells, suggesting inhibition of LDHB as a promising anticancer approach.