In this work, we report the facile synthesis of hydrophobic, flexible, and ultralightweight (rho(sponge) <= 17.3 mg/cm(3)) nanocellulose sponges using a novel and efficient silylation process in water. These functional materials with high porosity (>= 99%) are easily engineered by freeze-drying water suspensions of nanofibrillated cellulose (NFC), a natural nanomaterial isolated from renewable resources, in the presence of methyltrimethoxysilane sols of various concentrations. Microscopic and solid state nuclear magnetic resonance analyses reveal that the sponges are composed of a three-dimensional cellulosic network of thin sheets and nanofilaments, covered by polysiloxanes. Compared with conventional inorganic porous materials, the silylated NFC sponges display an unprecedented flexibility with a maximal shape recovery corresponding to 96% of the original thickness after 50% compression strain. The sponges also combine both hydrophobic and oleophilic properties and prove to be very efficient in removing dodecane spills from a water surface with an excellent selectivity and recyclability. Finally, the sponges can collect a wide range of organic solvents and oils with absorption capacities up to 100 times their own weight, depending on the density of the liquids. This versatile functionalization method opens up new opportunities for the design of novel advanced functional biomaterials with controlled properties.