--- - |2- Abstract Breast cancer is the most frequent cause of cancer in women. In the current study, transition metal ruthenium was complexed with flavonoid chrysin to evaluate the chemotherapeutic potential of this compound in Michigan Cancer Foundation‐7 (MCF‐7) human mammary cancer cell line and 7,12‐dimethylbenz(α)anthracene‐induced mammary cancer in female Sprague–Dawley rats. The characterizations of the complex were accomplished through UV–visible, NMR, IR, Mass spectra, and XRD techniques and antioxidant activity was assessed by DPPH, FRAP, and ABTS methods. In vitro studies included cell viability, cell cycle analysis, DNA fragmentation, and marker analysis by western blot analysis and found that complex treatment suppressed cell growth‐induced cell cycle arrest and enhanced the induction of apoptosis in cancer cells. Moreover, complex treatment modulated signaling pathways including mTOR, VEGF, and p53 in the MCF‐7 cells. Acute and subacute toxicity was performed in rats to determine the therapeutic doses. Breast cancer in rats was initiated by the administration of 7,12‐dimethylbenz(α)anthracene (0.5 mg/100 g body weight) via single tail vein injection. The histopathological analysis after 24 weeks of carcinogenesis study depicted substantial repair of hyperplastic lesions. Immunohistochemical analysis revealed upregulation of Bax and p53 and downregulation of Bcl2 proteins and TUNEL assay showed an increase in apoptotic index in ruthenium–chrysin‐treated groups as compared to the carcinogen control. Our findings from the in vitro and in vivo study support the continued investigation of ruthenium–chrysin complex possesses a potential chemotherapeutic activity against breast cancer and was efficient in reducing hyperplastic lesions in the mammary tissues of rats by inducing apoptosis. - Journal of Cellular Physiology, EarlyView.