MASTL (microtubule-associated serine/threonine kinase-like) is a critical kinase modulating mitotic entry. In this study, we investigated the mechanism of its dysregulation in breast cancer and its involvement in cisplatin resistance in ER+ breast cancer cells. Data mining in Kaplan-Meier Plotter showed that high MASTL expression was associated with worse distant metastasis free survival (DMFS) and relapse free survival (RFS) in ER+ breast cancer patients. In TCGA breast cancer cohort (TCGA-BRCA), MASTL was strongly co-upregulated with E2F8. High E2F8 expression was also strongly associated with unfavorable DMFS and RFS in ER+ breast cancer patients. Promoter scanning in JASPAR Database showed that the MASTL promoter region has a highly possible E2F8 binding site upstream the TSS site. The following western blot, dual luciferase assay and ChIP-qPCR validated this binding site. In MCF-7 cells, E2F8 overexpression alleviated cisplatin induced cell apoptosis by shortening G2/M arrest and promoting mitotic entry, the effect of which was largely canceled by inhibiting MASTL. Therefore, we infer that E2F8 can shorten cisplatin induced G2/M arrest by promoting MASTL mediated mitotic progression in ER+ breast cancer cells. These findings might help to explain why high MASTL or high E2F8 expression is associated with worse RFS in ER+ breast cancer.