The present study evaluated the efficacy of fennel seed methanolic extract (FSME) for its antioxidant, cytotoxic, and antitumor activities and for its capacity to serve as a nontoxic radioprotector in Swiss albino mice. We also assessed the natural antioxidant compounds of FSME for use in industrial application. Cytotoxic activity of FSME was evaluated in a mouse model of Ehrlich ascites carcinoma (EAC) and on different types of human cell lines in vitro. The safety and optimum dose of FSME were determined. FSME, 100 mg/kg, was injected intraperitoneally into mice bearing EAC before the mice were exposed to three 2-Gy doses of gamma irradiation. After 30 days, mice were fasted for 18 hours and then sacrificed to observe the lifespan of EAC-bearing hosts. Malondialdehyde (MDA), catalase activity, glutathione content, and total protein in serum, liver tissue, and ascitic fluid were determined. Iron, total iron-binding capacity, transferrin, and ferritin were also evaluated in serum. The data showed the presence of different types of compounds in FSME, such as flavonoids, terpenoids, alkaloids, phenols, and sterols; estragole (71.099%) was the predominant alcohol, gallic acid was the phenolic compound (18.895%), and L-limonene was the most prevalent monoterpene hydrocarbon (11.967%). The mean±standard deviation 50% inhibitory concentrations were 50±0.03 μg/mL for the MCF7 breast cancer cell line and 48±022 μg/mL for the Hepg-2 liver cancer cell line. The significant increase in MDA levels and the significant decrease in catalase activity and glutathione content in liver and tumor tissue in mice bearing EAC were ameliorated after FSME administration. In contrast, total protein content was increased in ascitic fluid. Serum iron was inversely proportional to the levels of ferritin and transferrin and total iron-binding capacity. Administration of FSME before irradiation exerted a cytoprotective effect against gamma irradiation, as manifested by a restoration of the MDA level, catalase activity, and GSH content to near-normal levels. In conclusion, FSME may have remarkable anticancer potential against a breast cancer cell line (MCF7) and liver cancer cell line (Hepg-2). It also showed strong free radical-scavenging activity (100%). Thus, FSME may reduce oxidative stress and protect mouse cells from damage caused by reactive oxygen species. In addition, it could be used as a safe, effective, and easily accessible source of natural antioxidants to improve the oxidative stability of fatty foods during storage. FSME also exhibited an antitumor effect by modulating lipid peroxidation and augmenting the antioxidant defense system in EAC-bearing mice with or without exposure to radiation.