Eutrophication, primarily driven by excessive ammonium nitrate and phosphate loading into aquatic ecosystems, poses a serious threat to water quality and biodiversity. In recent years, the valorization of agro-residues via pyrolysis to produce biochar has emerged as a sustainable and low-cost strategy for nutrient pollution control. This review explores the potential of biochar derived from agricultural waste as an efficient adsorbent for excess nutrient remediation from aqueous environments. The study highlights the physicochemical properties of biochar that influence its adsorption performance, such as surface area, porosity, functional groups, and inherent mineral content. Unmodified biochars generally exhibit limited nutrient removal efficiency; however, their adsorption performance can be significantly enhanced through surface modifications. These modifications include physical and chemical treatments, incorporation of metals and metal hydroxides, as well as the development of mineral/clay-based and other composite biochars, all of which improve surface functionality and affinity toward target nutrients. Exceptional adsorption capacity was recorded for PO₄³ ⁻ with lanthanum-loaded tobacco biochar (666.67 mg/g), outperforming other reported agro-residue-based derived biochars. Most systems followed pseudo-second-order kinetics and Langmuir isotherms, confirming chemisorption with monolayer coverage. Thermodynamic analysis revealed predominantly spontaneous adsorption, with both exothermic and endothermic processes, and regeneration tests showed stability for up to six cycles without significant performance loss. This review underscores the dual environmental benefit of converting agro-residue into value-added adsorbents while simultaneously addressing eutrophication, advocating for a circular and sustainable approach to water pollution management.