A facile gaseous CO2 mediated solid‐to‐solid transformation principle is adopted to insert additional CO32− anions into the thin single‐crystal nanosheets of Bi2O2CO3, which is built of periodic arrays of intrinsic CO32− anions and (Bi2O2)2+ layers. The additional CO32− anions create abundant defects. The Bi2O2CO3 nanosheets with rich interlayer CO32− exhibit superior electronic properties and charge transfer kinetics than the pristine single‐crystal 2D Bi2O2CO3 and display enhanced catalytic activity in photocatalytic CO2 reduction reaction and the photocatalytic oxidative degradation of organic pollutants. This work thus illustrates interlayer engineering as a flexible means to build layered 2D materials with excellent properties.