Dysregulation of the Notch-RBPJ (recombination signal-binding protein of immunoglobulin kappa J region) signaling pathway has been found associated with various human diseases including cancers; however, precisely how this key signaling pathway is fine-tuned via its interactors and modifications is still largely unknown. In this study, using a proteomic approach, we identified F-box only protein 42 (FBXO42) as a previously unidentified RBPJ interactor. FBXO42 promotes RBPJ polyubiquitination on lysine-175 via lysine-63 linkage, which enhances the association of RBPJ with chromatin remodeling complexes and induces a global chromatin relaxation. Genetically depleting FBXO42 or pharmacologically targeting its E3 ligase activity attenuates the Notch signaling-related leukemia development in vivo. Together, our findings not only revealed FBXO42 as a critical regulator of the Notch pathway by modulating RBPJ-dependent global chromatin landscape changes but also provided insights into the therapeutic intervention of the Notch pathway for leukemia treatment.