Background <p>Leafy amaranths (<i>Amaranthus</i> spp.) are promising traditional food and nutritional sources in Asia and Africa. They hold excellent value as underutilized, climate-resilient vegetable crops with versatile potential for food and nutritional security. Majority of the vegetable amaranth germplasm cultivated by Indian farmers consists of local landraces, and has not been subjected to systematic characterization. Hence, comprehensive characterization of amaranth germplasm for bioactive, antioxidant, and antinutritional factors in contrasting environments is vital for accelerating breeding of vegetable amaranth to underscore its potential in fulfilling United Nations sustainable development goals (SDGs) of Zero Hunger (SDG2) and Climate Action (SDG13).</p> Results <p>A diverse panel of 57 vegetable amaranth germplasm was investigated for antioxidant activity, bioactive compounds and antinutritional factors across winter (2024–2025) and summer (2025) seasons. Field experiment was laid out in randomized augmented block design with three commercial checks. Seasonal variations revealed the influence of G × E interactions on antioxidant activity, bioactive compounds and concentration of antinutrients. Clear temporal trends were recorded for antioxidant capacities (DPPH, CUPRAC, FRAP), bioactive traits and anti-nutrients (oxalates and nitrates). Genetic and multivariate analysis critically discriminated the germplasm and enabled the identification of potential parents for hybridization to exploit heterosis. Mahalanobis-D² cluster analysis suggested IC-0647405, IC-0647419, IC-0647408, IIHR-B-AM247 and IIHR-B-AM248 as a promising source of antioxidant phytochemicals, β-carotene, protein, vitamin C and phenolics with lower content of anti-nutrients. Pusa Kiran, IC-0647419 and IHR_B_AM248_3_2 exhibited lower concentration of oxalates, while IC-0647414, IC-0647411 and IIHR-B-AM237_5_14_3 depicted lowest accumulation of nitrates. High heritability and genetic advance estimates for studied traits indicated the scope of selective breeding in improvement of vegetable amaranth. Both synergistic and antagonistic trait associations were recorded between antioxidant and antinutrient traits.</p> Conclusion <p>Our study revealed substantial seasonal and genetic variations for antioxidant, bioactive and antinutritional traits in leafy amaranth germplasm, which is crucial for escalating crop breeding programmes. Results of genotype × environment interactions demonstrated the need of multi-environment, multi-year stability analysis for robust selection of multi-trait genotypes. The multivariate insights paved the way to breed amaranth cultivars with high nutritional quality and wider adaptability.</p>