A growing body of evidence suggests that the inflammatory NFκB pathway is associated with the progression of ER+ tumors to more aggressive stages. However, it is unknown whether NFκB is a driver or a consequence of aggressive ER+ disease. To investigate this question, we developed breast cancer cell lines expressing an inducible, constitutively active form of IκB kinase β (CA-IKKβ), a key kinase in the canonical NFκB pathway. We found that CA-IKKβ blocked E2-dependent cell proliferation in vitro and tumor growth in vivo in a reversible manner, suggesting that IKKβ may contribute to tumor dormancy and recurrence of ER+ disease. Moreover, coactivation of ER and IKKβ promoted cell migration and invasion in vitro and drove experimental metastasis in vivo. Gene expression profiling revealed a strong association between ER and CA-IKKβ–driven gene expression and clinically relevant invasion and metastasis gene signatures. Mechanistically, the invasive phenotype appeared to be driven by an expansion of a basal/stem-like cell population rather than EMT. Taken together, our findings suggest that coactivation of ER and the canonical NFκB pathway promotes a dormant, metastatic phenotype in ER+ breast cancer and implicates IKKβ as a driver of certain features of aggressive ER+ breast cancer.Significance: The canonical NFκB pathway promotes expansion of stem/basal-like cells and a dormant, metastatic phenotype in ER+ breast cancer cells. Cancer Res; 78(4); 974–84. ©2017 AACR.