Our recent study showed that SAG/RBX2 E3 ubiquitin ligase regulates apoptosis and vasculogenesis by promoting degradation of NOXA and NF1, and co-operates with Kras to promote lung tumorigenesis by activating NFκB and mTOR pathways via targeted degradation of tumor suppressive substrates including IκB, DEPTOR, p21 and p27. Here we investigated the role of Sag/Rbx2 E3 ligase in cellular senescence and immortalization of mouse embryonic fibroblasts (MEFs) and report that Sag is required for proper cell proliferation and Kras(G12D)-induced immortalization. Sag inactivation by genetic deletion remarkably suppresses cell proliferation by inducing senescence, which is associated with accumulation of p16, but not p53. Mechanistically, Sag deletion caused accumulation of Jun-B, a substrate of Sag-Fbxw7 E3 ligase and a transcription factor that drives p16 transcription. Importantly, senescence triggered by Sag deletion can be largely rescued by simultaneous deletion of Cdkn2a, the p16 encoding gene, indicating its causal role. Furthermore, Kras(G12D)-induced immortalization can also be abrogated by Sag deletion via senescence induction, which is again rescued by simultaneous deletion of Cdkn2a. Finally, we found that Sag deletion inactivates Kras(G12D) activity and block the MAPK signaling pathway, together with accumulated p16, to induce senescence. Taken together, our results demonstrated that Sag is a Kras(G12D)-cooperating oncogene required for Kras(G12D)-induced immortalization and transformation, and targeting SAG-SCF E3 ligase may, therefore, have therapeutic value for senescence-based cancer treatment.