Diosgenin, a natural steroidal saponin, has been reported to be found predominantly in fenugreek and has diverse biological properties. N-Methyl-N-nitrosourea (NMU) is a mammary gland-specific carcinogen that closely mimics human breast cancer in many aspects. The aim of this study was to investigate the anticarcinogenic property of diosgenin with reference to lipid peroxidation, status of antioxidants, and activities of marker enzymes against NMU-induced experimental mammary carcinogenesis. Breast cancer was induced in female Sprague Dawley rats by an intraperitoneal administration of a single dose of NMU (a concentration of 50 mg/kg body weight) diluted in 0.9% saline, and the rats were treated with oral diosgenin, 20 mg/kg body weight, for 45 days. The results were interesting, and the diosgenin treatment remarkably downregulated the peroxidation reaction and marker enzymes and extraordinarily enhanced the indigenous antioxidant defense system. The factor for this remarkable restoration might be due to the effect of the intervention strategy on the downregulation of the peroxidation reaction through the strong antioxidant nature, which ultimately reflected in the downregulation of marker enzyme activities. The histopathological study of breast and liver tissues inevitably confirms the biochemical changes. Thus, it can be concluded that diosgenin exhibits anticarcinogenic activity via reducing peroxidation reaction and marker enzymes through enhancing the intrinsic antioxidant defense system.