DNA double-strand breaks (DSBs) activate a signaling pathway known as the DNA damage response (DDR) which via protein–protein interactions and post-translational modifications recruit signaling proteins, such as 53BP1, to chromatin flanking the lesion. Depletion of the SET8 methyltransferase prevents accumulation of 53BP1 at DSBs; however, this phenotype has been attributed to the role of SET8 in generating H4K20 methylation across the genome, which is required for 53BP1 binding to chromatin, prior to DNA damage. Here, we report that SET8 acts directly at DSBs during the DNA damage response (DDR). SET8 accumulates at DSBs and is enzymatically active at DSBs. Depletion of SET8 just prior to the induction of DNA damage abrogates 53BP1's accumulation at DSBs, suggesting that SET8 acts during DDR. SET8's occupancy at DSBs is regulated by histone deacetylases (HDACs). Finally, SET8 is functionally required for efficient repair of DSBs specifically via the non-homologous end-joining pathway (NHEJ). Our findings reveal that SET8's active role during DDR at DSBs is required for 53BP1's accumulation.