Effective lineage-specific differentiation is essential to fulfilling the great potentials of human pluripotent stem cells (hPSCs). In this report, we investigate how modulation of medium pH and associated metabolic changes influence mesendoderm differentiation from hPSCs. We show that daily medium pH fluctuations are critical for the heterogeneity of cell fates in the absence of exogenous inducers. Acidic environment alone leads to cardiomyocyte generation without other signaling modulators. In contrast, medium alkalinization is inhibitory to cardiac fate even in the presence of classic cardiac inducers. We then demonstrate that acidic environment suppresses glycolysis to facilitate cardiac differentiation, while alkaline condition promotes glycolysis and diverts the differentiation toward other cell types. We further show that glycolysis inhibition or AMPK activation can rescue cardiac differentiation under alkalinization, and glycolysis inhibition alone can drive cardiac cell fate. This study highlights that pH changes remodel metabolic patterns and modulate signaling pathways to control cell fate.