Abstract The migration and invasion inhibitory protein MIIP is an inhibitor of cancer cell migration and invasion that inhibits breast tumorigenesis. In this case-control study, we evaluated the MIIP single nucleotide polymorphism (SNP) rs2295283 (codon 167, A>G, K>E) from 1,524 breast cancer patients and 1,592 age-matched controls for its association with breast cancer risk. SNP analysis included a validation set of 736 cases and 760 controls. Colony formation and cell migration assays were then conducted to functionally interrogate the genotype difference. When compared with the AA genotype, the combined AG + GG genotypes (167E) were associated with a significantly lower risk of breast cancer. In the test set, the protective effects of the AG + GG genotypes were more evident among participants with a family history of cancer. Further case series analysis revealed that the GG genotype was associated with reduced breast cancer susceptibility in cases of tumor size >2 cm and late clinical stage (II + III + IV). Colony formation assays showed that MIIP 167E (the G variant) was a more potent inhibitor of colony formation but not cell migration. These results suggest MIIP K167E as a functional genetic marker of breast cancer development and prognosis. Cancer Res; 70(3); 1024–32