Ethnopharmacological Relevance: Asteracantha longifolia Nees. (Syn. Hygrophila spinosa, Hygrophila auriculata) is a common weed growing in marshy and water logged areas. It is an important medicinal herb, widely distributed in India, and used by the local population for different medicinal purposes. In Ayurveda, the plant parts are used for the treatment of rheumatism, urinary tract infection, malaria, inflammation, diabetes, dysentery, jaundice, hepatic obstruction, pain, and decreased libido. Aim: The objective of the study is to explore the anticancer activity of the methanolic extract of A. longifolia on 7,12-Dimethylbenz (a) anthracene (DMBA)-induced mammary carcinogenesis in rats. Materials and Methods: The anticancer activity of the methanolic whole plant extract of A. longifolia was evaluated in Sprague Dawley rats in the mammary tumor induced by DMBA. The doses administered were 100, 200, and 400 mg/kg body weight by oral route. The extracts were administered for 20 consecutive days. The tumor size was determined before and after the administration of the plant extract and compared with the antitumor effect of the standard drug cyclophosphamide (CYC). After the experiment, the rats were sacrificed and subjected to histopathology and biochemical analysis. Results: A. longifolia extracts showed a significant (P < 0.05) decrease in the tumor size of DMBA-induced mammary tumor in mice. The extracts significantly (P < 0.05) decreased the levels of lipid peroxidation and significantly (P < 0.05) increased the levels of superoxide dismutase and catalase. Phytochemical screening revealed the presence of flavanoids, which are potent antioxidants. Conclusion: The results showed that the methanolic extract of A. longifolia was more effective in inhibiting the tumor growth and safe in DMBA-induced mammary tumor in rats as compared to the standard drug, cyclophosphamide.