Although chitosan-coated hydrostable papers are promising alternatives to non-degradable plastic-coated papers, an acidic chitosan solution is not eco-friendly. Therefore, we developed a non-acidic aqueous coating technique for preparing all-natural water-resistant papers using chitosan nanowhiskers (CSWs). As the CSWs disperse in water after synthesis, they form a hydrophobic layer on the paper after the coating process owing to the hydrophilic-to-hydrophobic self-assembly. Quantum simulations reveal that the repulsion between protonated amine groups contributes to water dispersibility; however, their interfibrillar interaction after coating is hardly dissociated by water. Together with soybean oil, the CSW- and oil-coated papers (C/O-papers) exhibit significantly higher water contact angles (95.8°) and absorption (6.0%) than control paper (21.9° and 91.9%, respectively). Moreover, the C/O-papers had a 16.0- and 9.1-fold higher mechanical modulus and strength (1498.5 and