Heavy metal pollution especially that of cadmium (II), poses serious threats to the environment and human health because of its toxicity and endurance. The synthesis of chitosan from crab shells, a fisheries waste, and its effectiveness as a biosorbent for the removal of Cd2+ from aqueous solutions are the subjects of this work. Crab shells were demineralised, deproteinised, and deacetylated to create chitosan, which was then characterised using FTIR to verify the functional groups (-NH₂, -OH) in charge of metal binding. The effects of pH, adsorbent dosage, beginning Cd2+ concentration, and contact time with adsorption behavior analyzed using Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm models, alongside kinetic and thermodynamic studies. The chitosan exhibited a maximum adsorption capacity of 714.3 mg/g at pH 8, with 92.5% removal efficiency, as described by the Langmuir isotherm model (R² = 0.99). Pseudo-second-order kinetics (R² = 0.9995) confirmed chemisorption as the dominant mechanism. Thermodynamic studies revealed an endothermic (ΔH° = 14.8 kJ/mol) and spontaneous (ΔG° = -6.1 to -8.2 kJ/mol) process, with efficiency increasing from 85.3% to 94.7% at 303–333 K. Compared to commercial chitosan, the crab shell-derived chitosan offers a cost-effective, sustainable alternative, aligning with circular economy principles. These results demonstrate its potential for large-scale wastewater treatment in areas affected by heavy metal contamination, and further study is advised to improve chitosan regeneration and modification for industrial use.