The present study evaluates the boron adsorption efficiency of modified carob kibble (Ceratonia siliqua L.), a low-cost, waste-derived biosorbent characterized by surface functional groups which enable interactions with charged ions. To enhance its adsorption properties, the carob kibble was modified using calcium chloride (CaCl2) at concentrations of 1M and 2M. Experimental conditions involved exposing 50 g/L of modified and non-modified carob kibble biomass (particle size: 0.025–0.106 mm) to boric acid (H3BO3) solutions at an alkaline pH of 11.50. At this pH, H3BO3 undergoes deprotonation, forming negatively charged borate ions (B(OH)4−), which dominate above pH 9. Adsorption assays were conducted using an initial boron concentration of 28 mg/L, simulating concentrations within the typical range found in industrial wastewater. The experiments were carried out for 5 h at 130 rpm at room temperature. The results show an improvement in boron removal with 2M CaCl2-modified carob kibble, achieving a removal efficiency of 92.5 ± 0.4% and an equilibrium adsorption capacity (qe, exp) of 0.525 ± 0.002 mg/g. In contrast, non-modified carob kibble biomass achieved a lower removal efficiency of 65.8 ± 0.3% and a qe, exp of 0.326 ± 0.001 mg/g. These findings highlight the potential of carob kibble, modified with environmentally abundant and low-cost materials like CaCl2, as a promising biosorbent for boron removal from aqueous media.