Although integrin α9 (ITGA9) is known to be involved in cell adhesion and motility, its expression in cancer and its role in tumor growth and metastasis remain largely unknown. Our study was designed to investigate the role of ITGA9 in triple-negative breast cancer (TNBC). ITGA9 expression in TNBC cells was knocked out (KO) using CRISPR/Cas9 technology. Four orthotopic mouse mammary xenograft tumor models coupled with cell culture studies were performed to determine the effect of ITGA9 depletion on TNBC tumor growth and metastasis and the underlying mechanism. Bioinformatics analysis showed that ITGA9 level is significantly higher in TNBC than other breast cancer subtypes, and higher ITGA9 level is associated with significantly worse distant metastasis-free survival and recurrence-free survival in TNBC patients. Experimentally, ITGA9 KO significantly reduced TNBC cell cancer stem cell (CSC)-like property, tumor angiogenesis, tumor growth and metastasis by promoting β-catenin degradation. Further mechanistic studies revealed that ITGA9 KO causes integrin-linked kinase (ILK) relocation from the membrane region to the cytoplasm, where it interacts with protein kinase A (PKA) and inhibits PKA activity leading to increased activity of glycogen synthase kinase 3 (GSK3) and subsequent β-catenin degradation. Overexpressing β-catenin in ITGA9 KO cells reversed the inhibitory effect of ITGA9 KO on tumor growth and metastasis. Furthermore, ITGA9 downregulation in TNBC tumors by nanoparticle-mediated delivery of ITGA9 siRNA drastically decreased tumor angiogenesis, tumor growth and metastasis. These findings indicate that ITGA9 depletion suppresses TNBC tumor growth and metastasis by promoting β-catenin degradation through the ILK/PKA/GSK3 pathway. © 2019 UICC.