Background <p>Poor prognosis of triple-negative breast cancer (TNBC) is owing to its intrinsic heterogeneity and lack of targeted therapies. Emerging evidence has characterized that targeting c-Myc might be a promising way to treat TNBC.</p> Methods <p>c-Myc knocked down TNBC cells were generated and the tamoxifen sensitivity was determined. Methylation-specific PCR analysis was used to detect the methylation status of ERα promoter, and c-Myc-mediated miRNA transcription was examined using chromatin immunoprecipitation and dual-luciferase reporter assays. The in vivo tamoxifen sensitivity was determined by mouse xenograft model.</p> Results <p>c-Myc knockdown in TNBC cells leads to the reactivation of ERα and consequent acquisition its sensitivity to tamoxifen. c-Myc depletion decreased the methylation in the promoter of ERα and DNMT1 was identified as the main executor. c-Myc knockdown-induced tamoxifen sensitivity was reversed by DNMT1 overexpression. The expression of miR-152-3p and miR-148a-3p was largely induced in c-Myc knockdown TNBC cells, and both miR-152-3p and miR-148a-3p could target DNMT1 to regulate its expression. c-Myc binds to the promoter regions of miR-152-3p and miR-148a-3p to exert transcriptional suppression. By suppressing miR-152-3p or miR-148a-3p expression using inhibitors, enhanced sensitivity to tamoxifen induced by c-Myc knockdown was partially reversed. In vivo xenograft tumor model demonstrated that c-Myc knockdown mildly inhibits&#xa0;the growth of tumor, and a dramatic decline was observed when administrated with tamoxifen combined with c-Myc knockdown.</p> Conclusion <p>Our study first illustrated that c-Myc knockdown in TNBC cells reactivate ERα expression in a miR-152/miR-148a-DNMT1-dependent manner, and brought new sights into treating TNBC using hormonal therapies.</p>