Thirty-eight-negative kinase 1 (TNK1) is a member of the ACK-family of nonreceptor tyrosine kinases and was originally cloned from CD34+/Lin-/CD38-hematopoietic stem/progenitor cells. The signaling pathways induced by TNK1 are largely unknown. Here, we report that expression and consequent activation of TNK1 enables tumor necrosis factor α (TNFα)-induced apoptosis by selectively inhibiting TNFα-induced activation of nuclear factor-κB (NF-κB). TNK1 has no effect on NF-κB DNA binding or the composition of the NF-κB complex; however, the kinase markedly prevents TNFα-induced NF-κB transactivation. TNK1 therefore acts as a novel molecular switch that can determine the properties of TNFα signaling and therefore cell death.