Bio-mediated synthesis of zinc oxide (ZnO) nanoparticles using plant extracts has been paid attention but still remains several challenges, e.g., alkaline addition during biosynthesis and photocatalytic effectiveness. Here, ZnO and silver (Ag)-doped ZnO at different ratios (0.5%, 1%, 3%, 5%, and 7%) were synthesized by an alkali-free method using Morinda citrifolia fruit extract. These materials were used as efficient photocatalysts for the tetracycline hydrochloride degradation. The impact of factors such as synthesis condition, Ag doping, pH, concentration, catalyst dosage, coexisting ions, and different light sources on the photocatalytic performance of green ZnO and Ag/ZnO nanoparticles was studied. The Ag-1%/ZnO composite exhibited the highest photocatalytic activity. The chief mechanisms involved in the photocatalytic process of Ag-doped ZnO nanoparticles were insightfully clarified through electrochemical and scavenging analyses. Active species including