The triple-negative breast cancer (TNBC), with a particularly poor prognosis, is increasingly recognized as heterogeneous in molecular signatures. MicroRNA expression profiles have been used for the classification and prognostication of breast cancer, numerous significantly upregulated microRNAs, i.e. miR-21, have been verified oncogenic in non-TNBCs. In present study, we determined the miR-21 levels in TNBC specimens, and TNBC cell levels in vitro, and then identified the role of miR-21 on tumor cell proliferation, apoptosis, and then identified PTEN as the possible target of the microRNA. It was shown that miR-21 expression is upregulated generally, and heterogeneous in TNBC specimens, posing a correlation with poor prognosis for TNBC patients. Further results demonstrated that the upregulated miR-21 promoted the tumor proliferation and inhibited cell apoptosis in vitro. And pro-apoptotic PTEN had been shown being targeted and downregulated. Therefore, our finding emphasized the oncogenic role of miR-21 in TNBC.