Up-regulated aerobic glycolysis is a hallmark of malignant cancers. Little is understood about the reasons why malignant tumors up-regulate glycolysis and acidify their microenvironment. Signaling pathways involved in glucose changes are numerous. However, the identity of the internal glucose signal remains obscure. In this review we address the question of the significance of vacuolar proton ATPase (V-ATPase) and its relationship to up-regulated glycolysis in tumors. We know that glycolysis is extremely sensitive to changes in pH. Importantly, the V-ATPase activity is sensitive to glucose availability. Therefore, we propose that pH acts as the glucose signal via the V-ATPase that responds to changes in intracellular pH and acts as a sensor. We hypothesize that the increase in glycolysis leads to intracellular acidification and activates the V-ATPase to maintain a more alkaline intracellular pH in tumors by up-regulating glycolysis. This review attempts to provide a comprehensive description of the current knowledge about the role of V-ATPase in cancer, highlighting its role as a key player in the pH signaling pathway.