Metastasis is an essential event for breast cancer (BC) progression even after initial surgery. The identification of patients with a high probability of metastasis at an early stage is particularly important in clinical practice and requires individualized treatment or early prevention. A retrospective study of 242 cases of ductal carcinoma in situ with microinvasion (DCIS-Mi), the first stage of invasive BC, was performed in this follow-up analysis. Of all patients, 8 developed metastases, and they were all included for further mechanistic studies with control group of 24 DCIS-Mi by matched-pair designing. By screening DCIS-Mi with different prognoses, we found that the DCIS-Mi that metastasized had significantly lower miR-135b-5p expression than the DCIS-Mi that did not. The function of miR-135b-5p was studied in vitro and in vivo invasion and metastasis assays. We also validated a novel target gene for miR-135b-5p, syndecan binding protein (SDCBP), and assessed the functional consequences of SDCBP by invasion assays. By checking different BC cell lines, a strong inverse correlation between miR-135b-5p and SDCBP expression was recorded. For the functional study, the inhibition of miR-135b-5p was accompanied by increased BC cell growth, epithelial-mesenchymal transition (EMT), migration and invasion in vitro. Interestingly, silencing SDCBP can reverse miR-135b-5p-dependent EMT and proliferation. In vivo studies demonstrated that the newly revealed miR-135b-5p/SDCBP axis increased cell proliferation, invasion and malignant transformation, as well as promoted metastasis in a xenograft tumor mouse model. Thus, our clinical patient cohort and functional data suggest that miR-135b-5p/SDCBP is a crucial determinant of BC metastasis at a very early stage. Our results may shed light on the importance of miR-135b-5p molecular diagnosis and prognosis, as well as the early prevention of BC for metastasis.