JunD is the most broadly expressed member of the Jun family and the AP-1 transcription factor complex. Primary fibroblasts lacking JunD displayed p53-dependent growth arrest, upregulated p19^Arf expression, and premature senescence. In contrast, immortalized cell lines lacking JunD showed increased proliferation and higher cyclinD1 levels. These properties are reminiscent of the effects of oncogenic Ras expression on primary and established cell cultures. Furthermore, JunD^−/− fibroblasts exhibited increased p53-dependent apoptosis upon ultraviolet irradiation and were sensitive to the cytotoxic effects of TNF-α. The antiapoptotic role of JunD was confirmed using an in vivo model of TNF-mediated hepatitis. We propose that JunD protects cells from senescence, or apoptotic responses to stress stimuli, by acting as a modulator of the signaling pathways that link Ras to p53.