Abstract <p>Synthetic polymers are derived primarily through chemical processing from fossil fuel-based raw materials. The extensive use of these non-biodegradable synthetic materials in pharmaceutical drug delivery, cosmetics, biomedical applications, and food packaging has caused significant environmental damage and poses potential health risks to living organisms. This calls for alternative, sustainable materials. Biopolymers, as sustainable and renewable polymeric sources, represent a viable alternative to overcome these issues. Recently, chitin, a highly abundant natural amino polysaccharide, and its derivatives have attracted researchers’ attention due to their outstanding properties, such as biodegradability, biocompatibility, non-toxicity, and low cost, which make them ideal candidates for pharmaceutical and biomedical applications. In this review, we selectively highlight the sources, extraction processes, biosynthesis, and properties of chitin and its derivatives. Furthermore, major research on chitin- and chitosan-based nanoformulations for drug delivery in various diseases, such as cancer, leishmaniasis, and microbial infections, is discussed. Similarly, nanoformulations for the delivery of numerous bioactive phytoconstituents, such as curcumin, rutin, ferulic acid, and quercetin, are also reviewed. Additionally, the immunogenicity, cytotoxicity, safety considerations, and clinical outcomes of various trials involving chitin and chitosan are summarized.</p>