Protein kinase C (PKC)-δ is a member of the PKC family. It has been implicated in tumor suppression as well as survival of various cancers. The aggressive malignancy of T lymphocytes known as adult T-cell leukemia (ATL) is associated with human T-cell leukemia virus type 1 (HTLV-1) infection. In this study, we show that HTLV-1-infected T cells are characterized by phosphorylation and nuclear translocation of PKC-δ. Expression of HTLV-1 regulatory protein Tax increased PKC-δ phosphorylation. Blockade of PKC-δ by rottlerin suppressed PKC-δ phosphorylation and inhibited cell viability in HTLV-1-infected T-cell lines and primary ATL cells. Rottlerin induced cell cycle arrest at the G1 phase and caspase-mediated apoptosis of HTLV-1-infected T cells. Rottlerin downregulated the expression of proteins involved in G1/S cell cycle transition, cyclin D2, CDK4 and 6, and c-Myc, resulting in dephosphorylation of retinoblastoma protein (pRb). Furthermore, rottlerin reduced the expression of important anti-apoptotic proteins (e.g., survivin, XIAP, Bcl-xL and c-FLIP) and Bcl-2 phosphorylation, and activated the pro-apoptotic protein Bax. Our results showed that permanent activation of nuclear factor-κB (NF-κB) by HTLV-1 Tax allows infected cells to escape cell cycle arrest and apoptosis and that PKC-δ mediates Tax-induced activation of NF-κB. Based on these findings, new therapies designed to target PKC-δ could be potentially useful in the treatment of ATL.