Natural organic matter, particularly humic acid (HA), is a major water contaminant due to its complex structure, strong metal-binding ability, and role as a precursor of toxic disinfection by-products formed during chlorination. This study developed a green and biodegradable ZnO–chitosan bio-composite (ZnO@Cs BC) for efficient HA removal from aqueous solutions. The composite was synthesized by dispersing ZnO within a chitosan matrix and characterized using FTIR, XRD, SEM/EDX, and TGA-DSC, confirming successful ZnO incorporation and enhanced structural and thermal stability compared with pristine chitosan. Batch adsorption experiments evaluated the effects of pH (3–11), contact time (10–180 min), adsorbent dosage (0.05–1.0 g), initial HA concentration (5–100 mg L⁻¹), and temperature (20–60 °C). The ZnO@Cs BC achieved over 90% HA removal across a broad pH range and a maximum removal efficiency of 99.8% under optimum conditions (pH 7, 25 °C, 120 min, 0.05 g adsorbent, and 5 mg L⁻¹ HA). Adsorption kinetics followed the pseudo-second-order model (R² ≈ 1), indicating chemisorption, while equilibrium data were best described by the Langmuir isotherm (R² = 0.99), with a maximum adsorption capacity of 4.535 mg g⁻¹. Thermodynamic analysis confirmed that adsorption was spontaneous (ΔG° < 0) and endothermic (ΔH° > 0). The composite retained approximately 85% of its adsorption capacity after five regeneration cycles, demonstrating excellent reusability. These results highlight ZnO@Cs BC as an effective, sustainable, and cost-efficient adsorbent for removing HA from water.