Humidity‐responsive ON/OFF switching of CO2 inclusion, associated with cooperative structural changes of heterogeneous cavities in crystalline peptide NiII macrocycles, 1⋅NO3, is demonstrated. The structural transformation between an “open form” and a “closed form” was caused by reversible water uptake into one of the cavities. The adsorption behavior of CO2 was studied with and without water vapor, combined with gas‐composition analysis by using gas chromatography. The studies revealed that the open form of 1⋅NO3 adsorbs CO2 gas up to 34 g mol−1, whereas the closed form adsorbs only a negligible amount of CO2 gas. Also, structural studies (X‐ray and IR spectroscopies) and investigation of water‐content dependence of CO2 adsorption revealed that the switching of gas inclusion occurred abruptly over a narrow range of water‐vapor pressure and was caused by the cooperative opening of vacant space to give the open form due to the structural transformation by water uptake. Let's cooperate! Humidity‐responsive ON/OFF switching of CO2 inclusion by using cooperative structural changes of heterogeneous cavities in crystalline peptide NiII macrocycles is demonstrated. The opening/closing of vacant cavities triggered by water uptake/release into the other cavities caused the switching of CO2 inclusion (see scheme).