Breast cancer (BC) and type 2 diabetes mellitus (T2DM) often co-occur and can contribute to poorer clinical outcomes after therapy compared to non-diabetic cases. It is assumed that glucose-lowering drugs like DPP4 inhibitors (DPP4-Is), e.g., sitagliptin, can affect the radiation responses of BC cells. In this study, parental and invasive (INV) MDA-MB-231 (triple-negative BC) and T47D (luminal A) cells were used. INV cells revealed a significant upregulation of DPP4/CD26 molecules compared to their parental counterparts. All investigated cells were treated with sitagliptin, ionizing radiation, or both. It was found that the specific inhibition of DPP4/CD26 molecules using sitagliptin can protect triple-negative BC cells from cytotoxic ionizing radiation or improve the radiation response in luminal A BC cells independently of their metastatic abilities. The protective and sensitizing effects of sitagliptin do not depend on the level of senescence and/or autophagy initiation after treatment.