Background: Genistein (GT) is an isoflavone phytoestrogen present in a number of plants. The chemical has been reported to have antioxidant, antigenotoxic, and cancer-preventive qualities; however, no studies against cisplatin (CP) have been reported. Objective: The main objective of the study is to determine the capacity of GT to inhibit the genotoxic and cytotoxic damage induced by CP in mouse, as well as its immunostimulant ability and its capacity to scavenge free radicals. Materials and Methods: We determined the effect of six doses of GT on the rate of sister chromatid exchanges (SCEs) and of micronuclei (MN) in mice administered with 5 mg/kg of CP. Besides, we determined its capacity to increase the amount of lymphocytes in mouse and to reduce oxidation with the 2,2-diphenyl-1-picrylhydrazyl assay. Results: Our results showed that GT (10–60 mg/kg) significantly decreased the frequency of SCE and of MN in mice. Furthermore, we also observed a moderate bone marrow cytotoxic correction of the damage induced by CP, as shown by an improvement in the rate of polychromatic erythrocytes. In addition, with 60 mg/kg, GT increased 69.6% the production of mouse lymphocytes over the control value throughout a 72-h trial. Moreover, the compound also showed a high capacity to trap free radicals (95.25%, with 250 μg/ml). Conclusion: Our results, therefore, established that GT is an effective cellular protective agent against the action of CP.