Purpose: To investigate the expression of miR-335 in triple-negative breast cancer (TNBC) and its effect on chemosensitivity. Methods: The expression of miR-335 in cancer tissues and adjacent tissues of 42 patients with TNBC who underwent mastectomy in our hospital was detected by qRT-PCR. Liposome was used to transfect miR-335 mimics (miR-335-mimic) and empty vectors (miR-NC) into cells of TNBC cell line MDA-MB-231, and untransfected cells were used as blank control cells (NC). Three groups of cells were cultured in culture Levbeit's medium supplemented with 2 mu mol/L paclitaxel, 5 mu mol/L cisplatin and 4 mu mol/L doxorubicin. Proliferation rate and apoptosis rate of tumor cells were measured by MTT assay and TUNEL assay 48 h after transfection. Results: The relative expression level of miR-335 in cancer tissues was significantly lower than that in adjacent tissues of TNBC patients (p<0.05). After paclitaxel, cisplatin and doxorubicin treatment, the proliferation and apoptosis rates of the three groups were statistically different (p<0.05). There was no significant difference in cell proliferation rate and apoptosis rate between NC group and miR-NC group (p>0.05), but the proliferation rate of cells was higher and apoptosis rate was lower in the NC group and miR-NC group than that in miR-335-mimic group (p<0.05). Conclusion: The expression level of miR-335 in cancer tissues of TNBC patients is lower than that in adjacent tissues. Overexpression of miR-335 can increase the sensitivity of tumor cells to paclitaxel, cisplatin and doxorubicin, and improve the effect of chemotherapy.