Water confined in nanoscopic pores is essential in determining the energetics of many physical and chemical systems. Herein, we report a recently discovered unconventional, reversible chemical reaction driven by water quantities in nanopores. The reduction of the number of water molecules present in the pore space promotes the hydrolysis of CO 3 2− to HCO 3 − and OH − . This phenomenon led to a nano‐structured CO 2 sorbent that binds CO 2 spontaneously in ambient air when the surrounding is dry, while releasing it when exposed to moisture. The underlying mechanism is elucidated theoretically by computational modeling and verified by experiments. The free energy of CO 3 2− hydrolysis in nanopores reduces with a decrease of water availability. This promotes the formation of OH − , which has a high affinity to CO 2 . The effect is not limited to carbonate/bicarbonate, but is extendable to a series of ions. Humidity‐driven sorption opens a new approach to gas separation technology.