Abstract: Under humid conditions, competitive adsorption between CO2 and H2O in porous adsorbents significantly limits the efficiency of adsorption-based carbon capture, particularly in direct air capture (DAC) processes. In this work, grand canonical Monte Carlo (GCMC) simulations were employed to systematically investigate the CO2/H2O competitive adsorption behavior in representative zeolites (AFI, FAU) and metal–organic frameworks (MgMOF-74, CuBTC), with thermodynamic analysis based on adsorption enthalpy, entropy, and Gibbs free energy changes. The results indicate that H2O preferentially occupies strong adsorption sites or forms hydrogen-bonded networks within pores under humid conditions, thereby suppressing CO2 uptake, with this effect being more pronounced at low temperatures and low CO2 partial pressures. Further analysis reveals that the adsorption selectivity between CO2 and H2O is governed by the synergistic effect of enthalpy and entropy rather than by enthalpy alone. Based on this, a thermodynamic criterion centered on ΔA (the difference in Gibbs free energy change) is established to quantitatively describe adsorption selectivity and its transition under varying temperature and humidity conditions. The findings demonstrate that simply increasing the adsorption enthalpy of CO2 is insufficient to overcome water competition; instead, tuning material polarity and leveraging temperature-dependent entropy contributions can effectively broaden the operational window for preferential CO2 adsorption, providing theoretical guidance for the design of water-resistant, high-selectivity adsorbents.