The stem bark of Ailanthus altissima is used in traditional medicine in Asia to treat a variety of diseases, including cancer. The aim of this study was to identify compounds with tumoricidal activity from A. altissima stem bark and to investigate their mechanisms of action. Among the 13 compounds isolated from the ethyl acetate fraction of A. altissima stem bark, the β-carboline alkaloid 9-hydroxycanthin-6-one had potent cytotoxicity in all three ovarian cancer cell types examined. 9-Hydroxycanthin-6-one induced apoptosis through the activation of caspases-3, -8, and -9. 9-Hydroxycanthin-6-one increased the intracellular levels of reactive oxygen species (ROS), and pre-treatment with the antioxidant N-acetyl-l-cysteine (NAC) attenuated the pro-apoptotic activity of 9-hydroxycanthin-6-one. Additionally, 9-hydroxycanthin-6-one was found to decrease the expressions of MCP-1 and RANTES, major determinants of macrophage recruitment at tumor sites, in ovarian cancer cells. Treatment with 9-hydroxycanthin-6-one inhibited the levels of M2 phenotype markers and some cancer-promoting factors, such as MMP-2, MMP-9, and VEGF, in macrophages educated in ovarian cancer conditioned medium. Taken together, these data suggest that 9-hydroxycanthin-6-one isolated from A. altissima stem bark induces apoptosis in human ovarian cancer cells through the caspase- and ROS-dependent pathways and inhibits the activation of tumor-associated macrophages.