The molecular underpinnings behind malignant progression of breast cancer from a localized lesion to an invasive and ultimately metastatic disease are incompletely understood. Here， we report that F-box only protein 22 (FBXO22) plays a dual role in mammary tumorigenesis and metastasis. FBXO22 was upregulated in primary breast tumors and promoted cell proliferation and colony formation and xenograft tumorigenicity Surprisingly， FBXO22 suppressed epithelial-mesenchymal transition (EMT)， cell motility， and invasiveness and metastatic lung colonization Clinical data showed that expression levels of FBXO22 were associated with favorable clinical outcomes， supporting the notion that metastasis， rather than primary cancer， is the major determinant of the mortality of patients with breast cancer. Mechanistic investigations further revealed that FBXO22 elicits its antimetastatic effects by targeting SNAIL， a master regulator of EMT and breast cancer metastasis， for ubiquitin-mediated proteasomal degradation in a glycogen synthase kinase 3β phosphorylation-dependent manner. Importantly， expression of SNAIL rescued FBXO22-mediated suppression of EMT， cell migration， and invasion. A patient-derived tryptophan-to-arginine mutation at residue 52 (W52R) within the F-box domain impaired FBXO22 binding to the SKP1-Cullin1 complex and blocked FBXO22-mediated SNAIL degradation， thus abrogating the ability of FBXO22 to suppress cell migration， invasion， and metastasis. Collectively， these findings uncover an unexpected dual role for FBXO22 in mammary tumorigenesis and metastatic progression and delineate the mechanism of an oncogenic mutation of FBXO22 in breast cancer progression. These findings highlight the paradoxical roles of FBXO22 in breast cancer， as it promotes breast tumor cell proliferation but prevents EMT and metastasis. .