<p>This study focused on the synthesis and characterization of nickel-doped spinel ferrites (Ni_XMg_1−XAl_0.3Fe_1.7O₄ (<i>X</i> = 0, 0.5)) via the sol–gel method for the photocatalytic degradation of norfloxacin. Incorporating nickel significantly improved the material properties, decreasing the crystallite size from 33.43 nm to 28.28 nm (a 15.4% reduction) and increasing the specific surface area from 5.57 m²/g to 8.67 m²/g (a 55.8% increase). The optical bandgap narrows by 5.3%, from 2.85 eV to 2.70 eV, enhancing visible-light absorption. These tailored properties improve the charge-carrier separation efficiency and increase the generation of reactive oxygen species. Under visible light and optimized conditions (pH 7, 30 °C, 10 mg catalyst, 10 ppm norfloxacin), the nickel-doped ferrite achieved 98.43% degradation efficiency of norfloxacin within 50 min, significantly outperforming the undoped sample (60.54% removal efficiency). Mechanistic studies identified hydroxyl radicals (HO*) as the primary oxidative species, with degradation following pseudo-first-order kinetics (<i>R</i>² = 0.9861). The magnetic properties of the catalyst allowed for easy recovery (&gt;95% recovery rate), thereby addressing the challenges of practical implementation. This ternary-substituted ferrite synthesis method optimizes visible-light absorption, structural stability, and surface reactivity, offering a promising approach for efficient norfloxacin degradation.</p><p></p>