This paper presents the synthesis and photocatalytic performance of Ag/Rb:ZnO thin films deposited utilizing the sol–gel spin-coating methodology. Pure and Ag/Rb:ZnO films with varying dopant concentrations (1, 3, and 5 at%) were deposited on glass substrates and annealed at 500 °C. X-ray diffraction analysis confirmed a hexagonal wurtzite structure with no secondary phases. Increasing dopant concentration reduced crystallite size from 8.23 nm (pure ZnO) to 4.52 nm (5% doping) with increased lattice strain. Surface morphology showed aggregated irregular particles decreasing from 90.45 nm to 68.22 nm at 5% doping, with Zn, Ag, and Rb confirmed by EDS. Optical studies revealed a redshift in absorption, with the band gap narrowing from 3.28 eV to 3.25 eV (3% doping) due to localized states. Photocatalytic degradation of methylene blue under UV light showed 3% Ag/Rb:ZnO achieved the highest efficiency (∼49%) versus pure ZnO. These findings highlight the potential of Ag/Rb:ZnO thin films for wastewater treatment applications.