Prostate adenocarcinoma accounts for majority of prostate cancer cases, and it was found to be highly radioresistant. Gallic acid is a phenolic acid naturally occurring in many plants, reported to exhibit biological activities in eliminating cancer cell lines and xenografts. The purpose of this study is to review gallic acid as a potential radiosensitizer agent in prostate cancer treatment. Article search was conducted in PubMed, EBSCO, and Scopus. 11 studies using different cell lines including DU145, PC-3, LNCaP, and 22Rv1 xenograft of human prostate cancer were reviewed in this paper. Gallic acid acts as a radiosensitizer mainly by increasing caspase-3 and caspase-9 activation resulting in apoptosis, while also reducing intracellular CDKs, cyclins, and cdc25 phosphatases ultimately causing G2-M cell cycle arrest. Gallic acid has a potential to be a new radiosensitizer compound in prostate cancer treatment. Additional clinical studies using gallic acid derivatives with lower hydrophilicity are needed. © 2021 UPM Press. All rights reserved.