The hypoxic microenvironment which is present following irradiation has been proven to promote radiation-induced injury to normal tissues. Previous studies have demonstrated that telomerase reverse transcriptase (TERT) is regulated by hypoxia, and that it plays a protective role in the process of wound repair. However, its effects on radiation-induced injury remain unclear. In this study, we examined the effects of human TERT on irradiation-induced late rectal injury in fibroblasts under hypoxic conditions. We also performed in vivo experiments. The rectums of 5-week‑old female C57BL/6N mice were irradiated locally with a single dose of 25 Gy. We then examined the fibrotic changes using hematoxylin and eosin staining, and Masson's staining. The expression of hypoxia inducible factor-1α (HIF-1α) and TERT was analyzed by immunohistochemistry. In in vitro experiments, apoptosis, reactive oxygen species (ROS) production and the autophagy level induced by exposure to hypoxia were assayed in fibroblasts. The association between TERT, nuclear factor-κB (NF-κB) and the autophagy level was examined by western blot analysis. The antioxidant effects of TERT were examined on the basis of the ratio of glutathione to glutathione disulfide (GSH/GSSG) and mitochondrial membrane potential. Rectal fibrosis was induced significantly at 12 weeks following irradiation. The HIF-1α and TERT expression levels increased in the fibrotic region. The TERT‑overexpressing fibroblasts (transfected with an hTERT-expressing lentiviral vector) exhibited reduced apoptosis, reduced ROS production, a higher autophagy level, a higher GSH/GSSG ratio and stable mitochondrial membrane potential compared with the fibroblasts in which TERT had been silenced by siRNA. NF-κB was activated by TERT, and the inhibition of TERT reduced the autophagy level in the fibroblasts. These results demonstrate that TERT decreases cellular ROS production, while maintaining mitochondrial function and protecting the cells from hypoxia-induced apoptosis, which may thus attenuate the effects of irradiation-induced hypoxia on rectal injury following irradiation.