Ataxia telangiectasia mutated and Rad3-related (ATR), a vital member of the phosphatidylinositol 3-kinase-related kinase (PIKK) family, plays a critical role in the DNA damage response (DDR). Tumor cells with a loss of DDR function or defects in the ataxia telangiectasia mutated (ATM) gene are generally more dependent on ATR for survival, suggesting that ATR is an attractive anticancer drug target based on its synthetic lethality. Herein, we present a potent and highly selective ATR inhibitor,ZH-12(IC50= 0.0068 μM). It showed potent antitumor activity as a single agent or in combination with cisplatin in the human colorectal adenocarcinoma LoVo tumor xenograft mouse model. Overall,ZH-12may be a promising ATR inhibitor based on the principle of synthetic lethality and deserves further in-depth study.