Mutant K-RAS has been shown to have both tumor-promoting and -suppressing functions, and growing evidence suggests that the RASSF family of tumor suppressors can act as RAS apoptosis and senescence effectors. It has been hypothesized that inactivation of the RASSF1A tumor suppressor facilitates K-RAS–mediated transformation by uncoupling it from apoptotic pathways such as the Hippo pathway. In human lung tumors, combined activation of K-RAS and inactivation of RASSF1A is closely associated with the development of the most aggressive and worst prognosis tumors. Here, we describe the first transgenic mouse model for activation of K-RAS in the lung in a RASSF1A-defective background. RASSF1A deficiency profoundly enhanced the development of K-RAS–driven lung tumors in vivo. Analysis of these tumors showed loss of RASSF1A-uncoupled RAS from the proapoptotic Hippo pathway as expected. We also observed an upregulation of AKT and RALGEF signaling in the RASSF1A− tumors. Heterozygosity of RASSF1A alone mimicked many of the effects of RAS activation on mitogenic signaling in lung tissue, yet no tumors developed, indicating that nonstandard Ras signaling pathways may be playing a key role in tumor formation in vivo. In addition, we observed a marked increase in inflammation and IL6 production in RASSF1A-deficient tumors. Thus, RASSF1A loss profoundly affects RAS-driven lung tumorigenesis and mitogenic signaling in vivo. Deregulation of inflammatory pathways due to loss of RASSF1A may be essential for RAS-mediated tumorigenesis. These results may have considerable ramifications for future targeted therapy against RAS+/RASSF1A− tumors.Significance: A transgenic mouse model shows that suppression of RASSF1A dramatically enhances Ras-driven tumorigenesis and alters Ras signaling pathway activity.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/10/2614/F1.large.jpg. Cancer Res; 78(10); 2614–23. ©2018 AACR.