It is well recognized that murine double minute gene 2 (MDM2) plays a critical role in cell proliferation and inflammatory processes during tumorigenesis. It is also reported that MDM2 is expressed in glomeruli and involved in podocyte injury. However, whether MDM2 is implicated in renal fibrosis remains unclear. Here we investigated the role of MDM2 in tubulointerstitial fibrosis (TIF). By immunohistochemical staining and Western blotting we confirmed that MDM2 is upregulated in the tubulointerstitial compartment in patients with TIF and unilateral urethral obstruction (UUO) mice, which mainly originates from myofibroblasts. Consistently, in vitro MDM2 is increased in TGF-β1-treated fibroblasts, one of the major sources of collagen-producing myofibroblasts during TIF, along with fibroblast activation. Importantly, genetic deletion of MDM2 significantly attenuates fibroblast activation. We then analyzed the possible downstream signaling of MDM2 during fibroblast activation. p53-dependent pathway is the classic downstream signaling of MDM2, and Nutlin-3 is a small molecular inhibitor of MDM2-p53 interaction. To our surprise, Nutlin-3 could not ameliorate fibroblast activation in vitro and TIF in UUO mice. However, we found that Notch1 signaling is attenuated during fibroblast activation, which could be markedly rescued by MDM2 knockdown. Overexpression of intracellular domain of Notch1 (NICD) by plasmid could obviously minimize fibroblast activation induced by TGF-β1. In addition, the degradation of NICD is strikingly suppressed by PYR-41, an inhibitor of ubiquitin-activating enzyme E1, and proteasome inhibitor MG132. Taken together, our findings provide the first evidence that MDM2 is involved in fibroblast activation and TIF, which associates with Notch1 ubiquitination and proteasome degradation.