Nowadays, cancer presents a serious health problem worldwide. Oxidative stress has an important role in a variety of patholo gies, and the search for an effective and well-tolerated antioxidant agent continues. Although cancer radiotherapy confers sig nificant benefits, it causes remarkable tissue damage and subsequent apoptosis as a result of ionizing radiation by hydroxyl radicals (__ampersandsignbull;OH) that follow the radiolysis of water. There is evidence, but inconclusive indicating that antioxidant capabilities of molecular hydrogen (H2 ) alleviate this oxidative stress and subsequent cancer complications and decrease death rates. This article discusses current knowledge on the implications of H2 in the reduction of radiation therapy-induced adverse effects in cancer. It also highlights the outcomes of recent clinical trials. Relevant articles were identified through an up-to-date online search of PubMed, Medline, Scopus, Science Direct, PsycINFO, registered clinical trials, Google Scholar, and WHO database. The available evidence shows that H2 as a therapeutic antioxidant selectively reduces the cytotoxic oxygen radicals, in particular, __ampersandsignbull;OH, and may improve cancer conditions. Despite the non-specific mechanism of H2 , researchers have reported several positive effects of H2 in cancer therapy, including delayed cancer progression, alleviation of radiotherapy and chemotherapy complica tions, and improved efficacy of conventional therapy when used in combination with H2 . It seems that H2 has protective effects in the radiotherapy of many cancers. Nonetheless, there is a general scarcity of controlled human studies that investigate direct and longitudinal clinical and biochemical effects of H2 on key cancerous changes. Thus, further research is required.