Notch3 and GATA binding protein 3 (GATA-3) have been, individually, shown to maintain luminal phenotype and inhibit epithelial-mesenchymal transition (EMT) in breast cancers. In the present study, we report that Notch3 expression positively correlates with that of GATA-3, and both are associated with estrogen receptor-α (ERα) expression in breast cancer cells. We demonstrate in vitro and in vivo that Notch3 suppressed EMT and breast cancer metastasis by activating GATA-3 transcription. Furthermore, Notch3 knockdown downregulated GATA-3 and promoted EMT; while overexpression of Notch3 intracellular domain upregulated GATA-3 and inhibited EMT, leading to a suppression of metastasis in vivo. Moreover, inhibition or overexpression of GATA-3 partially reversed EMT or mesenchymal-epithelial transition induced by Notch3 alterations. In breast cancer patients, high GATA-3 expression is associated with higher Notch3 expression and lower lymph node metastasis, especially for hormone receptor (HR) positive cancers. Herein, we demonstrate a novel mechanism whereby Notch3 inhibit EMT by transcriptionally upregulating GATA-3 expression, at least in part, leading to the suppression of cancer metastasis in breast cancers. Our findings expand our current knowledge on Notch3 and GATA-3's roles in breast cancer metastasis.