The present work reports the synergistic photocatalytic effect of pure ZnO and plasmonic Ag decorated ZnO nanoparticles against Rhodamine B under visible light exposure. Here, the Cyperus pangorei plant extract was utilized as a potent green bioactive source for effective production of ZnO and Ag/ZnO nanoparticles. The plant phytochemicals have induced the stabilization of ZnO nanoparticles decorated with silver. The obtained photocatalyst was characterized from XRD, FTIR, UV-DRS, SEM with EDX, TEM, and XPS. The zero-valent silver ions decorated on the ZnO surface and Ag existence modified the structural, morphological, optical surface area and free radicals formation on the ZnO surface. The visible light-mediated photocatalyst was evaluated against Rhodamine B. The Ag⁰ implications on the ZnO surface suppressed the e–h pair recombination and extend the radicals viability. Photocatalytic dye degradation of Rhodamine B was achieved 93% for Ag/ZnO at 120 min incubation. The Ag/ZnO nanoparticles’ rate constant of pseudo-first-order kinetics is 0.0181 min⁻¹. The holes, electrons and free radicals were demonstrated from quenching experiment. Holes are main quencher for the enhanced photocatalytic activity. The photocatalyst optimum selectivity of dye and catalyst concentration is 10 mg and high sensitivity at pH = 10 for better catalytic activity. The Ag/ZnO nanoparticles were examined against Escherichia coli (E. coli) in different conditions. The bacterial activity of Ag/ZnO nanoparticles specifies the before and after photocatalyst in the bacterial system. The before and after photocatalysis confirmed the free radicals reduction and oxidation properties. The holes and electrons that enhanced the cell deaths were analyzed by optical density measurement. The embedding of plasmonic nanoparticles on the ZnO surface controlled the size, shape, morphology and improved the photocatalytic and biological activities. Based on the results, the Ag/ZnO nanoparticles can be a very promising candidate for developing antimicrobial resistance and water remediation products.