Radiotherapy has an important role in the comprehensive treatment of breast cancer. However, the clinical outcome of adjuvant radiotherapy may be limited due to intrinsic radioresistance, it is necessary to explore efficient radiosensitization methods that improve the clinical outcome of patients undergoing radiotherapy. The present study aimed to investigate whether the novel mechanistic target of rapamycin (mTOR) inhibitor Torin2 enhances the radiosensitivity of MCF‑7 breast cancer cells. A Cell Counting Kit‑8 (CCK‑8) assay was performed to measure the effect of Torin2 on cell proliferation, while clonogenic assays were employed to determine the effect of Torin2 in combination with radiation on the proliferation of MCF‑7 cells. The effect of Torin2 and/or radiation on the cell cycle was analyzed using flow cytometry. Furthermore, the protein expression of components of the phosphatidylinositol 3‑kinase/Akt/mTOR pathway, and the expression of proteins involved in DNA damage repair, was measured by western blot analysis. The results demonstrated that Torin2 exhibited a higher potency in MCF‑7 cells, while MDA‑MB‑231 cells were less sensitive to Torin2. Compared with irradiation alone, pretreatment with 20 nM Torin2 followed by irradiation resulted in an increased level of γ‑H2A histone family member X. Radiation induced the activation of the Akt/mTOR signaling pathway and upregulated the expression of phosphorylated (p)‑Akt473 and p‑eukaryotic translation initiation factor 4E binding protein 1 (4EBP1)37/46. Notably, pretreatment with Torin2 attenuated the radiation‑induced activation of the Akt/mTOR signaling pathway. In addition, Torin2 partially blocked the repair of double‑strand breaks induced by radiation by reducing the activation of ataxia telangiectasia‑mutated, and sensitized MCF‑7 cells to radiation. In conclusion, administration of Torin2 prior to irradiation enhanced the radiotherapeutic effect on breast cancer cells in vitro, and these results may provide a foundation for the rational use of combined therapy with irradiation and Torin2 for breast cancer in clinical practice.