Acquired resistance to tamoxifen (Tam) is a critical problem in breast cancer therapy. Therefore, new potential strategies for Tam-resistant breast cancer are needed recently. In this study, we synthesized a novel histone deacetylase (HDAC) inhibitor, MHY218, for the development of potent inhibitors of HDAC and evaluated its biological activities by monitoring the anticancer effects in Tam-resistant MCF-7 (TAMR/MCF-7) cells via in vitro and in vivo studies. MHY218 significantly inhibited the proliferation of TAMR/MCF-7 cells in a dose-dependent manner. The total HDAC enzyme activity was significantly inhibited, corresponding with inhibition of acetylated H3 and H4 expression in TAMR/MCF-7 cells. HDAC1, 4, and 6 expression levels were decreased in response to MHY218 treatment. Cell cycle analysis indicated that MHY218 induced G2/M phase cell cycle arrest. As expected, apoptotic cell death was observed in response to MHY218 treatment. Interestingly, levels of beclin-1 and LC3-II, the markers of autophagy, were increased in TAMR/MCF-7 cells treated with MHY218. The efficacy of MHY218 was also compared with that of SAHA in vivo in a xenograft model of nude mice bearing a TAMR/MCF-7 cells. MHY218 (10 mg/kg, twice a week for 21 days) completely inhibited tumor growth and MHY218 markedly inhibited the expression of proliferative cell nuclear antigen (PCNA) in tumor tissue. These results indicate that MHY218 can induce caspase-independent autophagic cell death rather than apoptotic cell death. The MHY218-induced autophagic cell death could be a new strategy in the treatment of Tam-resistant human breast cancer.