Human T-cell leukemia virus 1 (HTLV-1), an oncogenic retrovirus, and Notchl signaling, implicated in tumor formation and progression, are both associated with the development of adult T-cell leukemia (ATL). Here we explored the possibility of a mechanistic link between the two. We observed that the expression of Notch intracellular domain (NICD) was elevated in HTLV-1 infected cell lines. Knocking down of Notchl in ATL cells repressed cellular proliferation and tumor formation both in vitro and in vivo. As a mechanism for these actions, we found that Tax activated Notch1 signaling by prolonging the half-life of NICD. We then showed that Tax, NICD, and RBP-j kappa formed a ternary complex, that Tax enhanced the association of NICD with RBP-j kappa, and that Tax, NICD, and RBP-j kappa were bound to RBP-j kappa-responsive elements. Hence, our results suggest that HTLV-1 promotes cellular proliferation and tumor formation of All cells by modulating Notch signaling via a posttranslational mechanism that involves interactions between Tax, NICD, and RBP-j kappa. 2019 Elsevier Inc. All rights reserved.