The effect of chitosan additives on the mechanical and hydrophobic properties, and air permeability of paper sheets was investigated. As chitosan additives, molecular chitosan and nanochitosan were used. Molecular chitosan was obtained by dissolving chitosan in 1% acetic acid. Nanochitosan was obtained using a thermocatalytic destruction method. It has been established that chitosan additives improve the mechanical properties of paper sheets (tensile strength in dry and wet states, burst strength). In the case of molecular chitosan, tensile index in dry and wet states increases with increasing chitosan dosage till 2.5-3.0% and then remains constant. It can be explained by the fact that chitosan forms ionic bonds and polyelectrolyte complexes with hemicelluloses and cellulose. At the dosage of 2.5-3.0%, all possible ionic bonds are already formed and further chitosan can form only hydrogen bonds by amino groups with cellulose. In the case of micro-nanochitosan, micro-nanoparticles fill also the submicroscopic voids in the porous structure of paper and create additional bonds. Chitosan additives also improve the hydrophobic properties (decrease of water adsorption, and increase of wetting time) and air permeability of paper sheets.