Abstract: Background: In recent years, phytochemicals have gained international scientific recognition for their potential to prevent major health problems. Research on the chemical structures and pharmacological mechanisms of plant extracts with anticancer effects has received considerable attention. Work's objective: The present study aimed to evaluate the antitumor chemotherapeutic potential of Tongkat Ali (Eurycoma longifolia) extract against breast carcinogenesis in Swiss albino mice resulting from Ehrlich's ascites carcinoma (EAC) transplantation. Materials and Methods: The efficacy of Tongkat Ali (Eurycoma longifolia) extract against Ehrlich's ascites carcinoma was evaluated by monitoring tumor weight, size, and recurrence. Lipid peroxidation (MDA) and the biochemical profile associated with oxidative stress were studied, as well as the antioxidant profile, which included the activity of superoxide dismutase (SOD), glutathione reductase (GR), glutathione-s-transferase (GST), total antioxidant capacity (TAC), catalase, and CAT, as well as markers of hepatotoxicity and renal toxicity (creatinine, urea, and aminotransferases), and histopathological changes after treatment. The Results: Reduced oxidative stress, increased antioxidant levels, restored liver and kidney function, and inhibited tumor growth and neovascularization. Conclusion: Based on all the data obtained, our research indicates that treatment with Eurycoma longifolia extract provided strong chemotherapeutic efficacy and antioxidant defense against breast tumors implanted in EAC.