Breast cancer is one of the most common malignancies in women. Approximately 15% of the patients belong to the triple-negative breast cancer (TNBC) group, and have the disadvantage of not benefiting from currently available receptor-targeted systemic therapies. Some cancers in the TNBC group harbor defects in DNA double-strand break repair by homologous recombination (HR), such as BRCA1 dysfunction, and are hypersensitive to poly (ADP-ribose) polymerase (PARP) inhibition. However, only a small fraction of the tumors are BRCA-deficient, and this restricts the therapeutic utility of the PARP inhibitor monotherapy. Cyclin-dependent kinase 1 (CDK1) is necessary not only for BRCA1-mediated S phase checkpoint activation, but also for HR, because it phosphorylates BRCA1 for the efficient formation of BRCA1 foci. In this study, we showed that the combined inhibition of CDK1 and PARP in BRCA-proficient MDA-MB-231 breast cancer cells resulted in dramatically reduced cell growth compared to PARP inhibition alone. Mechanistic investigations revealed that this sensitivity appears to be mediated by sustained DNA damage and inefficient DNA repair triggering mitochondrial-mediated apoptosis as well as autophagy. Our results suggest that CDK1 inhibition represents a plausible strategy for expanding the utility of PARP inhibitors to BRCA-proficient breast cancers.