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Amaranthus viridis is a functional food due to its antioxidant activity. The aim of this study was to investigate the responses of photosynthesis, growth, and antioxidant properties in A. viridis to nitrogen (N) applications. A. viridis plants were cultivated under low N (LN), medium N (MN), and high N (HN), and harvested at the reproductive phase. The dry mass and plant height of A. viridis plants increased with elevated N, and the dry mass of HN was saturated. Net photosynthetic rate, stomatal conductance, and water-use efficiency in the leaves at HN were strengthened. Meanwhile, under HN, chlorophylls (Chl), their precursors, and degradation intermediates in the leaves were highly accumulated, and the minor route of Chl degradation pathway was induced dramatically. However, 2,2-diphenyl-1-picrylhydrazyl radical-scavenging, ferrous iron-chelating, and reducing power in the extracts were reduced under HN. Conclusively, an appropriate N application balanced the yield and antioxidant properties of A. viridis.