Most cellulose-based materials' manufacturing processes include processing this biopolymer in an aqueous medium. Sorption properties depend on cellulose supramolecular structure and nature of its change during moistening. Plenty of researchers' efforts have been directed to the development of scientifically sound and commercially reliable processes over the past decade for the cellulose fibers' dispersion in an aqueous medium. Therefore, it needs a more detailed study of the cellulose-water system components' interaction. This study presents the supramolecular structure and sorption properties of native cotton cellulose research results obtained by 1 H NMR relaxation, spectroscopy and sorption measurements. Hydrophilic properties of cellulose as an adsorbent are characterized, taking into account a porous system between its structural elements. We examine in detail water adsorption on the active surface of cellulose Iβ. We also demonstrate the approach for determining the entropy change in the first two layers of adsorbed water and estimate this value increased during adsorption. Cellulose moistening is accompanied by the decomposition of macrofibrils into microfibrils and is manifested in a crystallinity decrease and a specific surface area growth.