Abstract <p>This review presents a summary of the diverse range of chitosan applications that have been developed recently: from biomedicine, agrobiotechnology, biostimulation, and bioprotection to climate change adaptation, among others. The focus is on the functionalization and controlled optimization of key biopolymer parameters: degree of polymerization, degree of acetylation, distribution of acetylated blocks within the biopolymer chain, density, and cross-linking reagents. The development of chitosan-based systems for encapsulating drugs and vaccines described in this review improves precision drug delivery. The biologization of agriculture is reflected in this review through studies devoted to the use of chitosan as a protective agent against abiotic stresses, such as drought, high and low temperatures, and soil salinization. By focusing on new strategies for the combined use of chitosan and plant-associated soil bacteria, the use of chitosan nanoparticles demonstrates the expanding scope of biopolymer applications in the agricultural sector. The presented discussion of the mechanism of chitosan action on the immune system of plants provides a deeper understanding of how to achieve the targeted effects of bioprotection and biostimulation.</p>