Mdm2 directly regulates the p53 tumor suppressor. However, Mdm2 also has p53-independent activities, and the pathways that mediate these functions are unresolved. Here we report the identification of a specific association of Mdm2 with Mre11, Nbs1, and Rad50, a DNA double strand break repair complex. Mdm2 bound to the Mre11-Nbs1-Rad50 complex in primary cells and in cells containing inactivated p53 or p14/p19ARF, a regulator of Mdm2. Further analysis revealed that Mdm2 directly bound to Nbs1 but not to Mre11 or Rad50. Amino acids 198-314 of Mdm2 were required for Mdm2/Nbs1 association, and neither the N terminus forkhead-associated and breast cancer C-terminal domains nor the C terminus Mre11 binding domain of Nbs1 mediated the interaction of Nbs1 with Mdm2. Mdm2 co-localized with Nbs1 to sites of DNA damage following gamma-irradiation. Notably, Mdm2 overexpression inhibited DNA double strand break repair, and this was independent of p53 and ARF, the alternative reading frame of the Ink4alocus. The delay in DNA repair imposed by Mdm2 required the Nbs1 binding domain of Mdm2, but the ubiquitin ligase domain in Mdm2 was dispensable. Therefore, Nbs1 is a novel p53-independent Mdm2 binding protein and links Mdm2 to the Mre11-Nbs1-Rad50-regulated DNA repair response.