53BP1 is recruited to chromatin in the vicinity of DNA double-strand breaks (DSBs). We identify the nuclear kinesin, KIF18B, as a 53BP1-interacting protein and define its role in 53BP1-mediated DSB repair. KIF18B is a molecular motor protein involved in destabilizing astral microtubules during mitosis. It is primarily nuclear throughout the interphase and is constitutively chromatin bound. Our observations indicate a nuclear function during the interphase for a kinesin previously implicated in mitosis. We identify a central motif in KIF18B, which we term the Tudor-interacting motif (TIM), because of its interaction with the Tudor domain of 53BP1. TIM enhances the interaction between the 53BP1 Tudor domain and dimethylated lysine 20 of histone H4. TIM and the motor function of KIF18B are both required for efficient 53BP1 focal recruitment in response to damage and for fusion of dysfunctional telomeres. Our data suggest a role for KIF18B in efficient 53BP1-mediated end-joining of DSBs.