Carbon-based materials have attracted interest as high-performance catalysts for the aqueous-phase conversion of cellulose. The adsorption of β-glucans plays a crucial role in the catalytic performance of carbons, although the primary driving force and details of the adsorption process remain unclear. This study demonstrates that adsorption occurs at hydrophobic sites on the carbon surface and that hydrophilic groups are not involved. Analysis of adsorption temperature dependence also reveals that the entropy change associated with adsorption is positive. Our results indicate that adsorption occurs by entropically driven hydrophobic interactions in addition to CH–π hydrogen bonding. These same CH–π hydrogen bonds are also confirmed by DFT calculations. The adsorption of β-glucans on carbons is significantly stronger than the affinity between β-glucans. The adsorption equilibrium constants of β-glucans on carbons increase exponentially with increasing degrees of polymerization, which supports the theory of strong interactions between the carbon and the long β-glucans found in cellulose.