A new adsorbent was developed by integrating algae biomass (AG) into a chitosan (CN) matrix, followed by structural enhancement via crosslinking with pyromellitic dianhydride (PMDA) through a hydrothermal synthesis approach. This process resulted in the formation of a robust AG@CN-PMDA composite with improved physicochemical characteristics suitable for advanced adsorption applications. The AG@CN-PMDA composite was evaluated for its efficiency in removal of the cationic dye methyl violet 2B (MV 2B) from aqueous solution. The adsorption process was refined through the Box-Behnken design (RSM-BBD), evaluating three essential parameters: adsorbent dosage (A: 0.02–0.1 g/100 mL), pH (B: 4–10), and time (C: 5–20 min). The ideal conditions for attaining the best removal rate for MV 2B (86%) were determined based on the desirability function optimisation results, corresponding to 0.09 g/100 mL of AG@CN-PMDA, at a pH of 6.9 and time of 9.45 min. The adsorption isothermal analysis revealed a close fit between the experimental data of MV 2B adsorption and both the Temkin and Langmuir models, with the Temkin model showing a slightly better correlation. Furthermore, the adsorption kinetics are well-described by the pseudo-second-order model. The maximum adsorption capacity of AG@CN-PMDA was 162.3 mg/g at 25°C. The adsorption of MV 2B onto AG@CN-PMDA was spontaneous, endothermic, and entropy-driven as evidenced by negative ΔG° values. The binding of MV 2B dye onto the AG@CN-PMDA composite was facilitated through mechanisms such as hydrogen bonding, π–π stacking, and electrostatic attraction. These findings demonstrate that AG@CN-PMDA is an effective and sustainable adsorbent for the removal of cationic dyes from industrial effluents.