Resistance to irradiation (IR) remains a major therapeutic challenge in tumor radiotherapy. The development of novel tumor-specific radiosensitizers is crucial for effective radiotherapy against solid tumors. Here, we revealed that remodeling tumor tissue penetration via tumor-penetrating peptide internalizing arginine-glycine-aspartic acid RGD (iRGD) enhanced irradiation efficacy. The growth of 4T1 and CT26 multicellular tumor spheroids (MCTS) and tumors was delayed significantly by the treatment with IR and iRGD. Mechanistically, iRGD reduced hypoxia in MCTS and tumors, resulting in enhanced apoptosis after MCTS and tumors were treated with IR and iRGD. This is the first report that shows enhanced radiation efficacy by remodeling tumor-specific tissue penetration with iRGD, implying the potential clinical application of peptides in future tumor therapy.