Despite the clinical success of tamoxifen, its resistance remains a major challenge in breast cancer. Here we show that Aurora-A determines tamoxifen sensitivity by regulation of oestrogen receptor (ER)α. Ectopic expression of Aurora-A decreases and depletion of Aurora-A enhances tamoxifen sensitivity in ERα-positive breast cancer. Elevated Aurora-A was significantly associated with the recurrence of ERα-positive tumours. Notably, Aurora-A inhibitor MLN8237, which is currently in clinical trial, synergizes with tamoxifen and overcomes tamoxifen resistance. Furthermore, Aurora-A interacts with and phosphorylates ERα on serine-167 and -305, leading to increase in ERα DNA-binding and transcriptional activity. Elevated levels of Aurora-A are significantly associated with disease-free survival in ERα-positive but not ERα-negative breast cancers. These data suggest that Aurora-A has a pivotal role in tamoxifen resistance and ERα is a bona fide substrate of Aurora-A. Thus, Aurora-A represents a prognostic marker in ERα-positive tumour and a critical therapeutic target in tamoxifen-resistant breast cancer, and Aurora-A inhibitor could be used as either an independent or concurrent agent in tamoxifen-resistant tumour.