Accumulating data have shown the involvement of microRNAs in cancerous processes as either oncogenes or tumor suppressor genes. Here, we established miR-30a as a tumor suppressor gene in breast cancer development and metastasis. Ectopic expression of miR-30a in breast cancer cell lines resulted in the suppression of cell growth and metastasis in vitro. Consistently, the xenograft mouse model also unveiled the suppressive effects of miR-30a on tumor growth and distal pulmonary metastasis. With dual luciferase reporter assay, we revealed that miR-30a could bind to the 3'-untranslated region of metadherin (MTDH) gene, thus exerting inhibitory effect on MTDH. Furthermore, we demonstrated that silence of MTDH could recapitulate the effects of miR-30a overexpression, while overexpression of MTDH could partially abrogate miR-30a-mediated suppression. Of significance, expression level of miR-30a was found to be significantly lower in primary breast cancer tissues than in the paired normal tissues. Further evaluation verified that miR-30a was negatively correlated with the extent of lymph node and lung metastasis in patients with breast cancer. Taken together, our findings indicated miR-30a inhibits breast cancer proliferation and metastasis by directly targeting MTDH, and miR-30a can serve as a prognostic marker for breast cancer. Manipulation of miR-30a may provide a promising therapeutic strategy for breast cancer treatment.