K-ras is one of the most frequently mutated human oncogenes. Activation of K-ras can lead to either senescence or proliferation in primary cells. The precise mechanism governing these distinct outcomes remains unclear. Here we utilized a loss-of-function screen to assess the role of specific genes identified as potential key regulators of K-ras driven oncogenesis. Using this approach, we identify the transcription factor Wt1 as an inhibitor of senescence in primary cells expressing oncogenic K-ras. Deletion or suppression of Wt1 expression leads to senescence of primary cells expressing oncogenic K-ras under the control of the native promotor at physiological levels, but has no effect on cells expressing wild-type K-ras. Wt1 contributes to K-ras driven lung tumorigenesis in vivo and loss of Wt1 is specifically deleterious to human lung cancer cell lines that are dependent on oncogenic K-ras. Taken together, these observations reveal a novel role for Wt1 as a key regulator of the complex genetic network required for the oncogenic effect of the small GTPase K-ras.