Zinc oxide nanoparticles (ZnO NPs) were sustainably synthesized using Capparis decidua stem extract as a green capping and stabilizing agent, and their photocatalytic efficiency toward Rhodamine 6G (Rh 6G) degradation was evaluated. Three extract concentrations (2, 5, and 10 g per 200 mL) were employed to optimize synthesis and performance. The ZnO NPs were characterized by XRD, FTIR, TEM, UV-Vis spectroscopy, and N₂ adsorption–desorption analysis. Pristine ZnO NPs were obtained using low plant extract concentrations. TEM revealed spherical nanoparticles whose average diameter decreased from 40 to 24 nm with increasing extract concentration. The sample synthesized with the highest extract concentration exhibited the largest surface area (32.9 m² g⁻¹) and pore volume (0.039 cm³ g⁻¹), indicating the impact of extract concentration on the material texture. The optical band gap narrowed from 3.67 eV to 3.02 eV while increasing the extract concentration from 2 g/200 mL (sample 2Z) to 10 g/200 mL (sample 10Z). Under sunlight irradiation, the 2Z sample achieved the highest photocatalytic degradation efficiency (for Rh 6G) of ∼96% at the optimum pH of 6.5 (in 75 min), compared to ∼88.4% for the sample 10Z. The degradation followed first-order kinetics with a rate constant of 0.018 min⁻¹. Superoxide radicals (O₂˙⁻) were the primary reactive species governing the photocatalytic degradation of Rh 6G, with ˙OH and (h⁺) contributing secondary roles in the overall oxidation. These findings demonstrate the potential of Capparis decidua extract for the eco-friendly synthesis of efficient ZnO photocatalysts for environmental remediation.