Daxx is a regulatory protein for apoptosis signal-regulating kinase 1 (ASK1) which activates c-Jun NH2-terminal kinase (JNK) and p38 pathways in response to stressors such as tumor necrosis factor-alpha (TNFalpha). Here, we show that TNFalpha treatment induces the accumulation of Daxx protein through ASK1 activation by preventing its proteasome-dependent degradation. ASK1 directly phosphorylates Daxx at Ser(176) and Ser(184) and Daxx is required for the sustained activation of JNK. Tumorigenic mutant p53, which binds to Daxx and inhibits Daxx-dependent activation of ASK1, prevents Daxx phosphorylation and stabilization. When mutant p53 was depleted in cancer cells, Daxx was accumulated and the cell-killing effect of TNFalpha was restored. Our results indicate that Daxx not only activates ASK1 but also is a downstream target of ASK1 and that accumulated Daxx further activates ASK1. Thus, the Daxx-ASK1 positive feedback loop amplifying JNK/p38 signaling plays an important role in the cell-killing effects of stressors, such as TNFalpha. Tumorigenic mutant p53 disrupts this circuit and makes cells more tolerable to stresses, as its gain-of-function mechanism.