The impact of moisture on gas adsorption capacity reduction on coal has been well recognized, and empirical correlations are widely used to quantitatively evaluate the moisture effect. However, few studies are found on fundamental modeling of the moisture effect on gas adsorption capacity. In this work, two theoretical models on the basis of the extended Langmuir theory (EL-based) and the ideal adsorbed solution theory (IAS-based) were developed to account for the gas adsorption capacity with different pressures and moisture contents. With the parameters determined from the gas adsorption on dry samples and water adsorption on samples under atmospheric conditions, both models are able to predict the gas adsorption capacity under combined effects of gas pressure and moisture content. The models were verified through a set of experimental data from a coal sample from Australia, and they were further applied to describe the methane adsorption behavior on a coal sample from New Zealand. The results demonstrate that both models can reasonably predict the gas adsorption capacity on moist coal samples. Although one more parameter is required, the IAS-based model could match the experimental data with higher accuracy. The research findings in this work contribute to a better understanding of the fundamentals of gas adsorption characteristics on moist coal.