Maintenance of genomic stability is essential for normal organismal development and is vital to prevent diseases such as cancer. As genetic information is packaged into chromatin, it has become increasingly clear that the chromatin environment plays an important role in DNA damage response. However, how DNA repair is controlled by epigenetic mechanisms is not fully understood. Here we report the identification and characterization of lysine-specific histone demethylase 5B (KDM5B), a member of the JmjC domain-containing histone demethylases, as an important player in multiple aspects of DNA double-strand break (DSB) response in human cells. We found that KDM5B becomes enriched in DNA-damage sites after ironizing radiation and endonuclease treatment in a poly(ADP ribose) polymerase 1- and histone variant macroH2A1.1-dependent manner. We showed that KDM5B is required for efficient DSB repair and for the recruitment of Ku70 and BRCA1, the essential component of nonhomologous end-joining and homologous recombination, respectively. Significantly, KDM5B deficiency disengages the DNA repair process, promotes spontaneous DNA damage, activates p53 signaling, and sensitizes cells to genotoxic insults. Our results suggest that KDM5B is a bona fide DNA damage response protein and indicate that KDM5B is an important genome caretaker and a critical regulator of genome stability, adding to the understanding of the roles of epigenetics in the maintenance of genetic fidelity.