The flavonoid quercetin exhibits significant anticancer activities with few side effects. In the current study, we characterized TL-2-8, a quercetin derivative, as a novel anticancer agent in vitro and in vivo. Cell proliferation and viability were assessed using Cell Counting Kit-8 and CellTiter-Blue assay, respectively. Cell death was examined using PI staining or a TUNEL assay. Mitophagy was determined by measuring autophagic flux and by confocal imaging. Protein expression was examined by Western blotting. We found that TL-2-8 selectively inhibited the proliferation and decreased the viability of various cancer cells (the anti-proliferation IC50 values in MDA-MB-231, MDA-MB-468 and MCF-7 breast cancer cells at 72 h were 8.28, 8.56, and 9.58 μmol/L, respectively), and it displayed only slight cytotoxicity against normal MCF-10A and HEK-293 cells. In MDA-MB-231 and MDA-MB-468 breast cancer cells, TL-2-8 treatment induced the degradation of multiple Hsp90 client proteins without inducing Hsp70. TL-2-8 (3, 6, 12 μmol/L) dose-dependently inhibited the expression of AHA1, an activator of Hsp90 ATPase, and decreased Hsp90-AHA1 complex formation, leading to decreased Hsp90 chaperone function and reduced polo-like kinase 1 (PLK1) signaling. Consequently, impaired mitophagy was induced via the downregulation of lysosomal-associated membrane protein 2 (LAMP2). The in vivo anticancer effects of TL-2-8 were evaluated in an MDA-MB-231 breast cancer xenograft model, which was treated with TL-2-8 (25, 50, 100 mg·kg-1·d-1, po). Administration of TL-2-8 resulted in tumor growth inhibition rates of 37.9%, 58.9% and 70.9%, respectively, whereas quercetin treatment (100 mg·kg-1·d-1, po) produced only a lower tumor growth inhibition rate (49.5%). Furthermore, TL-2-8 treatment significantly extended the lifespan of mice bearing MDA-MB-231 breast cancer cell xenografts. Our results demonstrate that TL-2-8 induces significant cell death and immature mitophagy in breast cancer cells in vitro and in vivo via AHA1 abrogation.