The search for renewable alternatives to petroleum products for industrial applications is increasing. Each year, many tons of inedible plant material is produced, much of which is landfilled. Cellulose, the most abundant biopolymer in nature, present in lignocellulosic biomass wastes can be extracted and converted to nanometric scale. Nanocellulose (NC) has unique characteristics for the development of new materials: abundance, renewability and biodegradability, mechanical properties and its nanometric dimensions open a wide range of possible properties and applications to be discovered. One of the most promising uses of NC is as reinforcement of mechanical properties in polymeric bionanocomposites. This review aims to give a recent view on this emerging nanomaterial, focusing on lignocellulosic biomass wastes extraction procedures, and its application in new technological developments. The challenges and future opportunities of bionanocomposites reinforced with NC will be discussed, as well as the remaining obstacles to its valorization and use.