The DNA end-joining protein Ku70 is one of several proteins that inhibit apoptosis by sequestering the proapoptotic factor Bax from the mitochondria. However, the molecular mechanism underlying Ku70-dependent inhibition of Bax is not fully understood. Here, we show that the absence of Ku70 results in the accumulation of ubiquitylated Bax. Under normal growth conditions, Bax ubiquitylation promotes its degradation. Upon induction of apoptosis in wild-type cells, a significant reduction in the levels of ubiquitylated Bax was observed, whereas in Ku70(-/-) cells, the ubiquitylated Bax was robustly accumulated. Addition of recombinant Ku70 into a protein extract of Ku70(-/-) cells resulted in a decrease in the levels of ubiquitylated Bax, even in the presence of proteasome inhibitors. Moreover, an in vitro deubiquitylation assay demonstrated that recombinant Ku70 hydrolyzed polyubiquitin chains into monoubiquitin units. Thus, Ku70 regulates apoptosis by sequestering Bax from the mitochondria and mediating Bax deubiquitylation. These results shed light on the role of proteasome inhibitors as tumor suppressors.