<p>This study aims to synthesize a 4-hydroxybenzaldehyde-functionalized chitosan Schiff base/hydroxyapatite composite (CS-4HBA/HAP) as an advanced polymeric adsorbent. The CS-4HBA/HAP biocomposite was characterized by several techniques: Brunauer–Emmett–Teller (BET) analysis, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and field emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (FESEM–EDX). The adsorption performance of CS-4HBA/HAP on cationic dye (basic red 2, BR2) removal was investigated. The results showed that CS-4HBA/HAP had a BET surface area of 13.71 m²/g, an average pore width of 10.07 nm, and a total pore volume of 0.03455 cm³/g. With an average crystallite size of 15.18 nm, the CS-4HBA/HAP mostly displays polycrystalline characteristics. The Box–Behnken design (BBD) was employed to optimize the adsorption process parameters, which included the CS-4HBA/HAP dose (0.01–0.09 g), pH (4–10), and adsorption time (10–40 min). The adsorption kinetics complied with the pseudo-first-order (PFO), and the adsorption isotherms complied with the Freundlich model. The adsorption capability of CS-4HBA/HAP biocomposite was determined to be 237.62 mg/g, with the dye adsorption mechanism being due to a wide variety of interactions, including electrostatic, n–π, hydrogen bonding, Lewis acid–base interaction, and π–π interactions. This study introduces the Schiff base-CS-4HBA/HAP composite as an effective adsorbent for removing BR2 dye, indicating its applicability in cationic dye removal.</p> Graphical Abstract <p></p>