Although pterostilbene, a natural analog of resveratrol, has potent antitumor activity against several human cancer types, the possible inhibitory mechanisms against subtypes of human breast cancer with different hormone receptor and human epidermal growth factor receptor 2 (HER2) status remain unknown. We investigated the anticancer activity of pterostilbene using three subtypes of breast cancer cell lines. Pterostilbene treatment exhibited a dose-dependent antiproliferative activity, with the greatest growth inhibition observed in triple-negative MDA-MB-468 cells. Although pterostilbene arrested cell-cycle progression at the G(0)/G(1) phase regardless of breast cancer subtype, its apoptosis-inducing activity was highly apparent in MDA-MB-468 cells. Pterostilbene induced strong and sustained activation of extracellular signal-regulated kinase (ERK) 1/2, with concomitant cyclin D1 suppression and p21 up-regulation, and inhibited the phosphorylation of AKT and mammalian target of rapamycin (mTOR), followed by subsequent up-regulation of BAX without affecting B-cell lymphoma-extra large (BCL-xL). Oral administration of pterostilbene significantly suppressed tumor growth in nude mice xenotransplanted with MDA-MB-468 cells. These data suggest a potential role of pterostilbene for prevention and treatment of human breast cancer, especially of triple-negative breast cancer.