Tumor cells proliferate rapidly, and thus are frequently subjected to replication stress and the risk of incomplete duplication of the genome. Fragile sites are replicated late, making them more vulnerable to damage when DNA replication fails to complete. Therefore, genomic alterations at fragile sites are commonly observed in tumors. FRA16D is one of the most common fragile sites in lung cancer, however, the nature of the tumor suppressor genes affected by FRA16D alterations has been controversial. Here, we show that the ATMIN gene, which encodes a cofactor required for activation of ATM kinase by replication stress, is located close to FRA16D and is commonly lost in lung adenocarcinoma (LUAD). Low ATMIN expression was frequently observed in human LUAD tumors and was associated with reduced patient survival, suggesting that ATMIN functions as a tumor suppressor in LUAD. Heterozygous Atmin deletion significantly increased tumor cell proliferation, tumor burden and tumor grade in the LSL-KRasG12D; Trp53 F/F (KP) mouse model of LUAD, identifying ATMIN as a haploinsufficient tumor suppressor. ATMIN-deficient KP lung tumor cells showed increased survival in response to replication stress, and consequently accumulated DNA damage. Thus, our data identify ATMIN as a key gene affected by genomic deletions at FRA16D in LUAD.