Etoposide is widely used for cancer chemotherapy in the clinic. However, long-term etoposide treatment can lead to adverse effects or drug resistance. To improve the situation, we evaluated the therapeutic efficiency of etoposide combined with inhibitors of bromodomain and extraterminal (BET) family proteins, which have recently emerged as novel anti-cancer targets due to their critical roles in cancer development. Firstly, we showed BRD4, one of the main targets of BET inhibitors, was involved in DNA damage response (DDR) via the homologous recombination (HR) repair pathway. Then we found BRD4 deficiency and inhibition reduced ataxia-telangiectasia mutated (ATM) recruitment and activation by down-regulating the expression of MRN complex (MRE11-RAD50-NBS1) proteins. Subsequently, our data showed that BET inhibitors could sensitize cancer cells to etoposide treatment both in vitro and in vivo. In summary, BRD4 inhibitors enhance the anticancer effect of etoposide by suppressing the MRN-ATM axis in DDR, thus indicating the important value of these inhibitors as candidates for combination with etoposide in chemotherapy.