BACKGROUND An increasing number of studies have shown that miRNAs are commonly deregulated in human malignancies, but little is known about the function of miRNA-205 (miR-205) in human breast cancer. The present study investigated the influence of miR-205 on breast cancer malignancy. METHODS The expression level of miR-205 in the MCF7 breast cancer cell line was determined by quantitative (q)RT-PCR. We then analyzed the expression of miR-205 in breast cancer and paired non-tumor tissues. Finally, the roles of miR-205 in regulating tumor proliferation, apoptosis, migration, and target gene expression were studied by MTT assay, flow cytometry, qRT-PCR, Western blotting and luciferase assay. RESULTS miR-205 was downregulated in breast cancer cells or tissues compared with normal breast cell lines or non-tumor tissues. Overexpression of miR-205 reduced the growth and colony-formation capacity of MCF7 cells by inducing apoptosis. Overexpression of miR-205 inhibited MCF7 cell migration and invasiveness. By bioinformation analysis, miR-205 was predicted to bind to the 3' untranslated regions of human epidermal growth factor receptor (HER)3 mRNA, and upregulation of miR-205 reduced HER3 protein expression. CONCLUSION miR-205 is a tumor suppressor in human breast cancer by post-transcriptional inhibition of HER3 expression.