SELENIUM is an essential element for humans, therefore, adding it to plants is convenient for biofortification. Thus, the aim of this work was to analyze experimentally the ability of sodium selenite to increase the concentration of Se and modify the ANTIOXIDANT ACTIVITY in tomato plants. We used plants of the Toro hybrid variety and applied three treatments: 0, 2 and 5 mg L-1 of SELENIUM as sodium selenite (Na2SeO3) using an irrigation system. Three samplings 40, 80, and 120 days after transplantation and a quantification of the accumulation of SELENIUM and macronutrients in leaves, stems, and fruits were carried out. Plant height, stem diameter, firmness, and total solids of fruits and total dry matter were measured. The enzyme ACTIVITY of catalase, glutathione peroxidase, and superoxide dismutase was quantified. The results indicated positive effects of Se on agronomic variables of the plants and fruit quality; however, fruit production showed no significant differences. Se had no effect on the concentration of the macro and micronutrients. The addition of 5 mg L-1 of Se resulted in significant increases in the concentration of SELENIUM on a dry basis, reaching 20.4 mg g-1 in leaves, 52.3 mg g-1 in stems, and 35.8 mg g-1 in fruits.The increase in enzyme ACTIVITY in the fruits was enhanced by the application of 5 mg L-1 of Se: There was up to 352.7% more catalase ACTIVITY, 312.2% more glutathione peroxidase ACTIVITY, and 200.8% more superoxide dismutase ACTIVITY compared with the control.