Onion, the second most important horticultural crop in the world after tomatoes, is intensively grown and widely consumed in Slovakia. A three-year field small-plot experiment was established at an agricultural farm in Western Slovakia using a block method. The aim of this experiment was to examine the impact of modified soil nutrient supplementation: 140 kg N. ha-1, N:S (140:19 kg.ha-1) and N:S:Fe (140:19:5 kg.ha-1) on the total polyphenol content (TPC) in selected onion cultivars (white – cv. Pueblo, yellow – cv. Mundo, red – cv. Robin and Kamal). The study showed that the total polyphenol content in onion was affected both by cultivar type (genotype) and soil nutrient supplementation. The highest average content of polyphenols was found in red cultivars treated with 140 kg N. ha-1 i.e. cv. Robin and cv. Kamal - 1048.25 and 1068.68 mg.kg-1 of fresh matter, respectively. Consumption of vegetable cultivars with high content of polyphenols may contribute to an antioxidant-rich diet that leads to major health benefits.