The AP-1 transcription factor c-Jun has been shown to be essential for stress-induced apoptosis in several models. However, the molecular mechanisms underlying the proapoptotic activity of c-Jun are poorly understood. We identify the apoptosis-antagonizing transcription factor (AATF) as a novel nucleolar stress sensor, which is required as a cofactor for c-Jun-mediated apoptosis. Overexpression or down-regulation of AATF expression levels led to a respective increase or decrease in the amount of activated and phosphorylated c-Jun with a proportional alteration in the induction levels of the proapoptotic c-Jun target genes FasL and TNF-α. Accordingly, AATF promoted commitment of ultraviolet (UV)-irradiated cells to c-Jun-dependent apoptosis. Whereas AATF overexpression potentiated UV-induced apoptosis in wild-type cells, c-Jun-deficient mouse embryonic fibroblasts were resistant to AATF-mediated apoptosis induction. Furthermore, AATF mutants defective in c-Jun binding were also defective in inducing AP-1 activity and c-Jun-mediated apoptosis. UV irradiation induced a translocation of AATF from the nucleolus to the nucleus, thereby enabling its physical association to c-Jun. Analysis of AATF deletion mutants revealed that the AATF domains required for compartmentalization, c-Jun binding, and enhancement of c-Jun transcriptional activity were all also required to induce c-Jun-dependent apoptosis. These results identify AATF as a nucleolar-confined c-Jun cofactor whose expression levels and spatial distribution determine the stress-induced activity of c-Jun and the levels of c-Jun-mediated apoptosis.