Estrogen receptor β1 (ERβ1) downregulation occurs in many breast cancers, but the responsible molecular mechanisms remain unclear. Here, we report that levels of ERβ1 expression are negatively regulated by the microRNA miR-92. Expression analysis in a cohort of primary breast tumors confirmed a significant negative correlation between miR-92 and both ERβ1 mRNA and protein. Inhibition of miR-92 in MCF-7 cells increased ERβ1 expression in a dose-dependent manner, whereas miR-92 overexpression led to ERβ1 downregulation. Reporter constructs containing candidate miR-92 binding sites in the 3′-untranslated region (UTR) of ERβ1 suggested by bioinformatics analysis confirmed that miR-92 downregulated ERβ1 via direct targeting of its 3′-UTR. Our results define a potentially important mechanism for downregulation of ERβ1 expression in breast cancer. Cancer Res; 70(11); 4778–84. ©2010 AACR.