KRAS mutations are responsible for a quarter of all lung adenocarcinomas. However, the molecular mechanisms linking these mutations and their frequent secondary dosage amplification to tumor formation are still not fully understood. While ample evidence supports a crucial role for the MAPK pathway in tumor development, the primary effectors targeted by this pathway remain largely unexplored. Here we identify the transcriptional repressor Capicua (CIC) as a key target inactivated by KRAS/MAPK signaling in lung adenocarcinoma. We show that genetic loss of CIC recapitulates the phenotypic consequences of amplified KRAS signaling. Genetic disruption of CIC suppressed the requirement for Kras allelic imbalances and accelerated the transformation of bronchiolar Club cells. We also demonstrate that restoring CIC repressor activity impaired proliferation of CIC-deficient tumor cells and reverted resistance to MAPK pathway inhibitors. These results highlight the key role of CIC during lung tumor formation and suggest that selective pressure for effective CIC inactivation favors secondary amplification of KRAS/MAPK signaling in tumor cells.