Nanofibrilated cellulose (NFC) films have potential to replace synthetic polymers as flexible films for packaging. However, NFC is hydrophilic and water acts as plasticizer decreasing the stiffness of the films and reducing its barrier effectiveness against water vapor and oxygen. Here we describe the surface modification of cellulose films with blocked diisocyanates through a dipping and heating process not requiring the previous drying of the materials. The reactions were conducted at 170 °C for a few minutes during which deblocking led to a new urethane bond formation with NFC surface hydroxyl groups, thus hydrophobizing the films. A remarkable enhancement in water repellent properties was confirmed by water contact angles higher than 110° and water vapor transmission rate (WVTR) of 40 g/m2 day, which is very low when compared to similar materials, representing a reduction of 74% with respect to the non- modified films.