<p>Ex-situ doping of the semiconductor bismuth oxy bromide (BiOBr) by the green synthesized Ag nanoparticles is detailed in this work. BiOBr and Ag-doped BiOBr nanoparticles are synthesized using the co-precipitation technique. Ocimum tenuiflorum leaf extract is used as the reagent for reducing silver nitrate to silver nanoparticles for the preparation of Ag-doped BiOBr. The incorporation of Ag into the BiOBr lattice is confirmed by the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies. Mesoporous nature of the BiOBr and Ag-doped BiOBr nanoparticles was identified using Brunauer–Emmett–Teller (BET) analysis. The pore radius and surface area were found to increase as a result of Ag doping in BiOBr. The Ag-doped BiOBr nanoparticles delivers an excellent 100% degradation of Sulforhodamine B (SRB) dye within 1-min under direct sunlight. The prominent reactive species controlling the degradation of SRB were determined to be the e-, •OH^.and h +. From quenching studies, it was also found that O₂- does not have a substantial impact on the photocatalytic degradation rate.</p>