Sulfamethoxazole (SMX), which is a commonly used antibiotic, poses persistent risks to aquatic environments owing to its recalcitrance. Adsorption mediated by sludge-derived biochar is a beneficial strategy as an emergency sorbent for antibiotic pollution control. However, its application is limited by the low pore volume and small specific surface area (SSA) of the sorbent owing to the high ash content of sludge. In this study, CaCl₂-modified sewage sludge-derived biochar (CaSBC) was prepared via impregnation–pyrolysis followed by acid washing. The treatment increased the pore volume and SSA to 0.3015 cm³/g and 129.50 m²/g, respectively, providing additional accessible pores for adsorption. Furthermore, the modification induced changes in the surface charge and functional groups, which improved electrostatic attraction over a wide pH range and enhanced π–π electron donor–acceptor and hydrogen bonding interactions between CaSBC and SMX. Consequently, CaSBC exhibited an adsorption capacity of 22.30 ± 0.74 mg/g, which was 2.3 and 2.9 times higher than that of unmodified biochar and commercial activated carbon (AC), respectively. Furthermore, CaSBC exhibited stable adsorption in the pH range of 2–8 and in the presence of competing ions, and its estimated production cost was lower than that of AC by 27%. This study presents a practical and cost-effective method for the reusage of sludge as a value-added resource and the concurrent enhancement of SMX removal from wastewater.