Amaranth (Amaranthus spp.) is a pseudo-cereal with increasing agronomic and nutritional importance due to its high seed yield potential and valuable lipid composition. The aim of this study was to evaluate the effects of NPK fertilization level, cultivar, row spacing and their interactions on yield components, crude fat content, fat yield and fatty acid composition of amaranth seeds. A three-year field experiment (2016–2018) was conduct-ed in southeastern Poland using two cultivars (Rawa and Aztek), four levels of NPK fertilization and two row spacings (30 and 55 cm). The results showed that cultivar was the main factor differentiating seed yield, crude fat content, fat yield and fatty acid composition. The Aztek cultivar produced significantly higher seed yield and fat yield, whereas Rawa was characterized by a higher proportion of unsaturated fatty acids. Row spacing significantly affected plant density and yield structure, while increasing NPK fertilization enhanced seed and fat yield. Significant two-factor interactions were identified. The interaction between fertilization level and cultivar significantly affected palmitic (C16:0) and stearic (C18:0) acids, whereas the interaction between row spacing and cultivar influenced linoleic acid content (C18:2, n-6) and the n-6/n-3 fatty acid ratio. These results confirm that both quantitative and qualitative traits of amaranth seeds are determined by complex interactions between genetic and agrotechnical factors.