Nanocellulose, derived from renewable sources such as cellulose fibers, has garnered significant attention as a reinforcement material in the paper industry due to its exceptional mechanical properties, high surface area, and sustainability. This chapter explores the dual role of nanocellulose as an additive and coating agent for enhancing the physical, mechanical, and barrier properties of paper. As an additive, nanocellulose creates a dense, hydrogen-bonded network inside the fiber matrix, significantly increasing the internal bonding and tensile strength of paper. As a coating agent, nanocellulose can provide a dense, smooth layer that improves surface properties like gloss and printability while adding mechanical strength, air and water resistance properties. Moreover, nanocellulose can be used as a composite material with synthetic coating ingredients to improve the properties as paper coatings. It enhances mechanical properties such as durability and tensile strength while offering better air and moisture resistance. These developments pave the way for paper industries to be more environmentally friendly, which is required to meet the rising demand for eco-friendly and sustainable alternatives to nondegradable plastic films.