Triple-negative breast cancer (TNBC) is associated with high recurrence rates of metastasis and death. miR-509 has been reported to be a tumor suppressor in many cancers, but its effect in TNBC has not yet been identified. In this article, we explored the effects of miR-509 on the malignant phenotype of TNBC cells, including proliferation, apoptosis, migration, and invasion. We transiently transfected TNBC cells, Hs578T, with miR-509 mimic. Upon transfection, the expression of miR-509 was upregulated about 50-fold compared with cells transfected with scramble mimic. Overexpression of miR-509 inhibited cell proliferation, induced cell apoptosis, and suppressed cell invasion of Hs578T cells. Moreover, tumor necrosis factor-α (TNF-α) was involved in miR-509-mediated suppressive effects of TNBC cells, as being treated with TNF-α could partially abolish the suppressive effects of miR-509. Collectively, these data suggest that miR-509 could reverse the malignant phenotype of TNBC cells, probably by suppressing TNF-α.