Potassium hydroxide supported on natural zeolite clinoptilolite (CP) is found to be an efficient catalyst for the carboxymethylation of hemicellulose. Adsorption and dissociation of KOH on the surface of CP were investigated by pH measurement and theoretical modeling analysis using density functional theory. Indeed, the adsorption of KOH on the surface of the CP porous structure provides a cheap solid catalyst with increased basicity. Different ratios of KOH:CP were examined, and the 1:2 ratio gave the best results with almost two times higher efficiency than standard KOH solution, even with shorter reaction time and lower temperature. The carboxymethylated sample with KOH:CP (1:2) indicated less degradation than KOH as catalyst investigated by viscosity measurements. Thermal analysis of the original and modified hemicellulose indicated that the degree of substitutions of the sample calculated through thermal analysis was close to the results from the titration method.