IKKepsilon has recently been identified as a breast cancer oncogene. Elevated levels of IKKepsilon are associated with cell survival and growth. Here, we show that IKKepsilon interacts with and phosphorylates estrogen receptor alpha (ERalpha) on serine 167 in vitro and in vivo. As a result, IKKepsilon induces ERalpha transactivation activity and enhances ERalpha binding to DNA. Cyclin D1, a major target of ERalpha, is transcriptionally up-regulated by IKKepsilon in a phospho-ERalpha-Ser-167-dependent manner. Further, overexpression of IKKepsilon induces tamoxifen resistance, whereas knockdown of IKKepsilon sensitizes cells to tamoxifen-induced cell death. These data suggest that ERalpha is a bona fide substrate of IKKepsilon and IKKepsilon plays an important role in tamoxifen resistance. Thus, IKKepsilon represents a critical therapeutic target in breast cancer.