The molecular basis of anticancer and apoptotic effects of R-goniothalamin (GON), a plant secondary metabolite was studied. We show that induction of oxidative stress and reactivation of mutant p53 underlie the strong cytotoxic effects of GON against the breast cancer cells. While GON was not toxic to the MCF10a breast epithelial cells, the SKBR3 breast cancer cells harboring an R175H mutant p53 were highly sensitive (IC50 = 7.3 µM). Flow cytometry and other pertinent assays showed that GON-induced abundant reactive oxygen species (ROS), glutathione depletion, protein glutathionylation and activation of apoptotic markers. GON was found to conjugate with glutathione both in vitro and in cells and the product was characterized by mass spectrometry. We hypothesized that the redox imbalance induced by GON may affect the structure of the R175H mutant p53 protein, and account for greater cytotoxicity. Using the SKBR3 breast cancer and p53-null H1299 lung cancer cells stably expressing the R175H p53 mutant protein, we demonstrated that GON triggers the appearance of a wild-type-like p53 protein by using conformation-specific antibodies, immunoprecipitation, DNA-binding assays and target gene expression. p53 restoration was associated with a G2/M arrest, senescence, reduced cell migration, invasion and increased cell death. GON elicited a highly synergistic cytotoxicity with cisplatin in SKBR3 cells. In SKBR3 xenografts developed in nude mice, there was a marked tumor growth delay by GON alone and GON + cisplatin combination. Our studies highlight the impact of tumor redox-stress generated by GON in activating the mutant p53 protein for greater antitumor efficacy.