Nuclear factor-κB (NF-κB) is generally believed to be pro-tumorigenic. Here we report a tumor-suppressive function for NF-κB1, the prototypical member of NF-κB. While NF-κB1 downregulation is associated with high lung cancer risk in humans and poor patient survival, NF-κB1-deficient mice are more vulnerable to lung tumorigenesis induced by the smoke carcinogen, urethane. Notably, the tumor-suppressive function of NF-κB1 is independent of its classical role as an NF-κB factor, but instead through stabilization of the Tpl2 kinase. NF-κB1-deficient tumors exhibit ‘normal’NF-κB activity, but a decreased protein level of Tpl2. Reconstitution of Tpl2 or the NF-κB1 p105, but not p50 (the processed product of p105), inhibits the tumorigenicity of NF-κB1-deficient lung tumor cells. Remarkably, Tpl2-knockout mice resemble NF-κB1 knockouts in urethane-induced lung tumorigenesis. Mechanistic studies indicate that p105/Tpl2 signaling is required for suppressing urethane-induced lung damage and inflammation, and activating mutations of the K-Ras oncogene. These studies reveal an unexpected, NF-κB-independent but Tpl2-depenednt role of NF-κB1 in lung tumor suppression. These studies also reveal a previously unexplored role of p105/Tpl2 signaling in lung homeostasis.