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BACKGROUND Leonotis nepetifolia (L.) R. Br. (Lamiaceae) is a shrub traditionally used to alleviate inflammatory conditions. OBJECTIVE The present study aimed at investigating the biological activity of methanolic nontransformed and transformed Rhizobium rhizogenes root extracts from L. nepetifolia against human melanoma cells. METHODS Cytotoxicity and genotoxicity properties, the impact on topoisomerase I activity, and proapoptotic activity were evaluated by the MTT test, comet assay, topoisomerase I assay, and fluorescence-activated cell sorting analysis. Moreover, the expressions of p53 were examined by qPCR and Western blot analysis. Docking studies were conducted to assess the potential interactions of the identified phytochemicals with the p53 binding protein Mdm-2, and computational analyses exhibited their antioxidant potential. RESULTS Both extracts showed cytotoxic potential against human melanoma cells, but generally the activity was more potent for transformed roots than untransformed (IC50 760 μg/mL and 980 μg/mL, respectively). A similar effect was revealed during the evaluation of genotoxic and proapoptotic properties. Moreover, the expression of p53 was also found to be increased after extract treatment. The most dominant identified compounds in both extracts were as follows: (+)- catechin, p-coumaric acid, m-coumaric acid, and (+)-rosmarinic acid. Docking studies and computational analysis showed that (+)-rosmarinic acid possesses the highest binding affinity to the p53 binding protein, Mdm-2, and exhibits the best antioxidant property from the most commonly identified phytochemicals. CONCLUSION Our findings revealed the potential of L. nepetifolia transformed root extract as a source of bioactive compounds with cytotoxic, genoto.