Adult T-cell leukemia/lymphoma (ATLL) is an aggressive disease caused by infection with human T-cell leukemia virus type 1 (HTLV-1). Successful treatment is limited by resistance to chemotherapies. Therefore, there is an urgent need to develop novel effective strategies. Artesunate (ART), a widely used antimalarial compound, has been shown to exert cytotoxicity. Here, we aimed to assess the anti-ATLL activities of ART and to elucidate the possible molecular mechanisms involved in this effect. Compared with uninfected T cells, HTLV-1-infected T-cell lines were sensitive to ART-induced cytotoxicity. ART caused cell cycle arrest at G and/or G/M phases, which was associated with decreased expression of cyclin dependent kinase 1/2/4/6, cyclin B1/D2/E and c-Myc, and increased expression of p21. ART-induced apoptosis corresponded to activation of caspase-8/9/3; decreased expression of Bcl-xL, Bcl-2, myeloid cell leukemia-1, survivin, X-linked inhibitor of apoptosis protein and cellular inhibitor of apoptosis 1/2; and increased expression of Bak. ART increased intracellular reactive oxygen species and activation of the DNA damage marker γ-H2AX. Moreover, ART-induced cytotoxicity was partly reversed by treatment with a reactive oxygen species scavenger, iron chelator, and necroptosis or ferroptosis inhibitor, suggesting the involvement of caspase-dependent and -independent lethal pathways. These effects were correlated with inhibition of nuclear factor-κB and activator protein-1 signaling through dephosphorylation of IκBα, IκB kinase (IKK) α and IKKβ, and decreased expression of JunB and JunD. Importantly, intraperitoneal injection with ART lowered tumor burden in an ATLL murine model. These preclinical results provide a rationale for evaluating the efficacy of ART in patients with ATLL.