This review examines the chemical modifications of chitin and chitosan fibers and filaments, emphasizing their potential for diverse applications. It discusses a variety of modifications, including amidoximation, acetylation, partial deacetylation, N-maleation, N-naphthaloylation, N-halamination, grafting, oxidation, phosphorylation, sulfation, N-acylation, Schiff base formation, quaternization, nucleophilic substitution, hydroxyethylation, and ureidation transformations. These modifications significantly enhance the properties of chitin and chitosan, improving their solubility, dispersibility, hydrophobicity, environmental stability, mechanical strength, antimicrobial activity, and hemostatic capabilities, among other characteristics. By strategically tailoring these biopolymers through chemical modifications, novel materials with enhanced functionalities can be synthesized. These advanced materials are crucial for developing functional products with broad applications across various fields, spanning from biomedicine to sustainable composites.