Ran-binding protein M (RanBPM) is a nucleocytoplasmic protein previously implicated in various signaling pathways, but whose function remains enigmatic. Here, we provide evidence that RanBPM functions as an activator of apoptotic pathways induced by DNA damage. First, transient expression of RanBPM in HeLa cells induced cell death through caspase activation, and in the long-term, forced expression of RanBPM impaired cell viability. RanBPM COOH-terminal domain stimulated the ability of RanBPM to induce caspase activation, whereas this activity was negatively regulated by the central SPRY domain. Second, small interfering RNA-directed knockdown of RanBPM prevented DNA damage-induced apoptosis, as evidenced by the marked reduction in caspase-3 and caspase-2 activation. This correlated with a magnitude fold increase in the survival of RanBPM-depleted cells. Following ionizing radiation treatment, we observed a progressive relocalization of RanBPM from the nucleus to the cytoplasm, suggesting that the activation of apoptotic pathways by RanBPM in response to ionizing radiation may be regulated by nucleocytoplasmic trafficking. Finally, RanBPM downregulation was associated with a marked decrease of mitochondria-associated Bax, whereas Bcl-2 overall levels were dramatically upregulated. Overall, our results reveal a novel proapoptotic function for RanBPM in DNA damage-induced apoptosis through the regulation of factors involved in the mitochondrial apoptotic pathway.