Water-repellent all-cellulose nanocomposite (ACNC) was made using the silane coupling agent dodecyltriethoxysilane. The ACNC, consisting of cellulose nanofibers incorporated into a cellulosic matrix, was prepared by reducing crystallinity in the surface of the cellulose nanofibers using DMAc/LiCl solvent, followed by applying pressure and drying. The ACNC surface turned from hydrophilic to hydrophobic by silane coupling treatment. X-ray photoelectron spectroscopy (XPS) measurements confirmed the existence of the silane coupling agent on the surface after treatment. As silane concentration increased, the water contact angle of treated ACNC increased, to a maximum value of 93 degrees. Thermogravimetric analysis and atomic force micrographs showed a multilayered treatment. The silane treated ACNC showed lower water absorption compared to that of untreated one. The silane coupling treatment was also found to increase the mechanical performance of ACNC. Creation of a hydrophobic surface reduces the potential damage associated with water in outdoor applications of ACNC.