Natural products have inspired man for millennia. Their rich biological, biochemical, and structural diversity has prompted scientists to isolate, understand, and use them in different areas, including in biological applications. This is the case of the biopolymers that are originally extracted from vegetal (cellulose) and animal waste resources or by microorganisms. This chapter addresses chemical surface modifications of biopolymers-based nanoforms, namely, nanocellulose, including bacterial cellulose (BC) and nanochitin, for biological applications. These biopolymers provide a sustainable solution to the need for new (bio)materials in biological applications due to their unique biological properties like low toxicity, biocompatibility, biodegradability, and bioactivity. Nonetheless, further chemical functionalization has been advanced in order to optimize their intrinsic properties and/or generate novel functions to develop new materials, in particular, functional bionanocomposites. These modifications are crucial in determining the potential of these materials in biological applications such as in food technology, pharmaceutical and biomedical industry, and in the biotechnology field.