Parietin is an orange anthraquinone secondary metabolite found in the top layer of the upper cortex of some lichens such as Xanthoria elegans. In the present investigation, parietin was characterized using high-performance liquid chromatography (HPLC) alongwith photodiode-array detection and HRMS (retention time: 6.7 min; UV λmax: 265.3, 286.7 and 434 nm; [M+H]+m/z: 284). Cyanobacterial cells are severely damaged by ultraviolet radiation (UVR), which mainly affects their lipids, proteins, DNA, and photosynthetic efficiency. This reduces the survival of cyanobacteria and ultimately causes cell death. Hence, the photoprotective properties of parietin against UV-irradiated Synechocystis sp. PCC 6803 cells were evaluated. UV-screening effects of parietin with regard to photosynthetic pigments, phycocyanin content (PC), lipid peroxidation, photosynthetic performance and quenching coefficients (FV/FM, rETR and Y(NPQ)) were determined. In addition, in vivo ROS, antioxidative enzyme (SOD and CAT) and cell viability using flow cytometry (FCM) were evaluated for the first time. The results show that various dosages of parietin have effective photoprotective activity, neutralizing the harmful effects of UVR in the cyanobacterium Synechocystis sp. PCC 6803. Because of its strong UV-absorbing ability and non-toxic characteristics, parietin may be preferably used in the pharmaceutical and cosmetic industries. Keywords: Antioxidative enzymes; Cell viability; Flow cytometry; Parietin; Photoprotective; Ultraviolet radiation.