Gastric cancer is a worldwide health problem. Chemotherapy and radiotherapy are of great importance in the management of advanced gastric cancer. However, their therapeutic efficacy is limited by off-target side effects. Peptide-drug conjugates (PDCs) are a novel strategy for tumor-targeted drug delivery to overcome the existing drug resistance mechanisms and improve antitumor effects. Kita-Kyushu lung cancer antigen 1 (KK-LC-1) is exclusively expressed in several types of cancer including gastric cancer, representing a promising target for drug delivery. Here, we suggested KK-LC-1 as a potential target for PDC design for the first time and reported the first KK-LC-1-targeting PDC product 1131-MMAE, which is composed of a KK-LC-1-targeting peptide and an antimitotic drug conjugated by an enzymatically cleavable linker. We observed that 1131-MMAE could be efficiently endocytosed by KK-LC-1 positive gastric cancer cells for subsequent drug release and arrest the cell cycle at the most radiosensitive G2/M phase. We demonstrated that 1131-MMAE could significantly delay tumor growth with reduced toxicity than free drugs as a monotherapy. We further confirmed that 1131-MMAE was also a potent radiosensitizer. 1131-MMAE could selectively enhance the radiation response of KK-LC-1 positive tumor cells and achieve improved tumor control when combined with low-dose radiation. Overall, our study proposed an optimized therapeutic regimen for precision chemoradiotherapy, which has translational potential in multiple types of cancer.