Exploring low‐energy reaction pathway of catalytic biomass conversion can lead to wider application and the achievement of sustainability objectives. Since glucose dehydrogenation to gluconic acid and H2 is a cost‐effective alternative to glucose oxidation, this study aims to elucidate its mechanism. The detection of lactone as an intermediate indicates that cyclic glucose reacts directly via its hemiacetal group–ring opening is not involved; that is, cyclic glucose is dehydrogenated to lactone, which is further hydrolyzed to gluconic acid. The source of hydrogen is confirmed to be from glucose and water by Isotope tracing analysis. Density function theory calculations demonstrate that Hemiacetal Dehydrogenation Pathway (this work) is less energy intensive than Ring‐opening Oxidation Pathway (previous works). This study provides a new dehydrogenation strategy to produce gluconic acid and H2 from biomass under mild conditions.

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