ZAS3 is a large zinc finger transcription repressor that binds the ϰB-motif via two signature domains of ZASN and ZASC. A loss-of-function study showed that lack of ZAS3 protein induced accelerated cell proliferation and tumorigenesis. Conversely, gain-of-function studies showed that ZAS3 repressed NFϰB-activated transcription by competing with NFϰB for the ϰB-motif. Based on these observations, we hypothesize that ZAS3 promotes apoptosis by interrupting anti-apoptotic activity of NFϰB. Here, we present evidence that upon TNFα stimulation, ZAS3 inhibits NFϰB-mediated cell survival and promotes caspase-mediated apoptosis. The inhibitory effect of ZAS3 on NFϰB activity is mediated by neither direct association with NFϰB nor disrupting nuclear localization of NFϰB. Instead, ZAS3 repressed the expression of two key anti-apoptotic genes of NFϰB, TRAF1 and TRAF2, thereby sensitizing cells to TNFα-induced cell death. Taken together, our data suggest that ZAS3 is a tumor suppressor gene and therefore serves as a novel therapeutic target for developing anti-cancer drugs.