To detect and repair damaged DNA, DNA-damage-response proteins need to overcome the barrier of condensed chromatin to gain access to DNA lesions. ATP-dependent chromatin remodelling is one of the fundamental mechanisms used by cells to relax chromatin in DNA repair^,. However, the mechanism mediating their recruitment to DNA lesions remains largely unknown. BRIT1 (also known as MCPH1) is an early DNA-damage-response protein that is mutated in human primary microcephaly^,,,,. Here we report a previously unknown function of BRIT1 as a regulator of the ATP-dependent chromatin remodelling complex SWI–SNF in DNA repair. After damage to DNA, BRIT1 increases its interaction with SWI–SNF through ATM/ATR-dependent phosphorylation on the BAF170 subunit. This increase in binding affinity provides a means by which SWI–SNF can be specifically recruited to and maintained at DNA lesions. Loss of BRIT1 causes impaired chromatin relaxation as a result of decreased association of SWI–SNF with chromatin. This explains the decreased recruitment of repair proteins to DNA lesions and the reduced efficiency of repair in BRIT1-deficient cells, resulting in impaired cell survival after DNA damage. Our findings therefore identify BRIT1 as a key molecule that links chromatin remodelling with response to DNA damage in the control of DNA repair, and its dysfunction contributes to human disease.