Recombination signal binding protein for immunoglobulin kappa J region (RBPJ) is encoded by the RBPJ gene in human and is an integral part of the Notch signaling pathway. It has been shown that the Notch receptor family and its related ligands are deregulated in several cancers. Moreover, RBPJ is found to play a vital role in immune cell function, including T cells, B cells, macrophages, and dendritic cells (DCs), and can influence the prognosis of cancers such as hepatocellular carcinoma. RBPJ has several available binding sites that favor the occupation of inhibitors and make it a potential target for clinical application. An example of a clinical application of RBPJ inhibitors is fidaxomicin, an FDA-approved drug, that is used for breast cancer growth by blocking RBPJ-related pathways. It is suggested that intervening RBPJ expression might be a therapeutic approach for treating human cancers, such as leukemia, prostate cancer, hepatocellular carcinoma, and lung cancer, and selective inhibition of RBPJ-mediated Notch signaling may be a valuable therapeutic target in cancer immunotherapy. However, it is noteworthy that current evidence regarding the safety and efficacy of using RBPJ inhibitors as a therapeutic option for patients with cancers is limited and more research is needed. This chapter aims to provide an overview of the role of RBPJ in the NOTCH signaling pathway, immunotherapy of various cancers, and its relationship with the immune system.