Cell cycle regulatory pathways in breast cancer are incompletely described. Here, we report an important role in estrogen receptor alpha (ERalpha)-positive breast cancer cells for the protein kinase C1 (PKC1)-interacting protein RBCK1 in supporting cell cycle progression by driving transcription of ERalpha and cyclin B1. RBCK1-depleted cells exhibited increased accumulation in G(2)-M phase of the cell cycle, decreased proliferation, and reduced mRNA levels for ERalpha and its target genes cyclin D1 and c-myc. Chromatin immunoprecipitation revealed that ERalpha transcription is associated with RBCK1 recruitment to the ERalpha promoter, suggesting that transcriptional regulation is one mechanism by which RBCK1 affects ERalpha mRNA levels. G(2)-M phase arrest was mediated independently from reduced ERalpha levels, instead associated with transcriptional inhibition of the key G(2)-M regulator cyclin B1. In breast tumor samples, there was a positive correlation between levels of RBCK1, ERalpha, and cyclin B1 mRNA levels. Our findings suggest that RBCK1 regulates cell cycle progression and proliferation of ERalpha-positive breast cancer cells by supporting transcription of ERalpha and cyclin B1.