The development of sustainable heterogeneous catalysts from agricultural waste offers an environmentally friendly strategy for advanced wastewater treatment. In this study, a novel waste-derived watermelon rind/magnetite (WMR/Fe₃O₄) composite was successfully fabricated as a magnetically recoverable catalyst for the synergistic adsorption–photo-Fenton degradation of SPR Red dye as an azo dye from aqueous solutions. The novelty of this work lies in integrating lignocellulosic watermelon rind with Fe₃O₄ nanoparticles to simultaneously enhance pollutant adsorption, catalytic oxidation, and catalyst recovery through a simple, low-cost preparation route. The synthesized composite was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), confirming the successful formation of crystalline Fe₃O₄ nanoparticles uniformly distributed over the biomass support. The effects of operational parameters, including catalyst dosage, hydrogen peroxide concentration, pH, dye concentration, and temperature, were systematically investigated. Under the optimum operating conditions (pH 2.9, catalyst dosage 1.0–1.2 g L⁻1, and H₂O₂ concentration 430 mg L⁻1), the photo-Fenton system achieved 96% SPR dye degradation within approximately 50 min, significantly outperforming adsorption and conventional heterogeneous Fenton processes. Kinetic analysis demonstrated that the degradation followed a pseudo-first-order model, while thermodynamic analysis confirmed a spontaneous and exothermic reaction with a low activation energy of approximately 20 kJ mol⁻1. Reactive oxygen species scavenging experiments identified hydroxyl radicals (•OH) as the dominant oxidizing species responsible for dye degradation. Moreover, the catalyst retained more than 75% of its initial activity after multiple reuse cycles, demonstrating excellent stability and magnetic recoverability. Process optimization using Box–Behnken response surface methodology accurately predicted the optimum operating conditions with excellent agreement between experimental and predicted values. These findings demonstrate that the WMR/Fe₃O₄ composite is a low-cost, sustainable, and highly efficient heterogeneous photo-Fenton catalyst with promising potential for practical treatment of dye-contaminated industrial wastewater.