Terminal heat stress during anthesis and grain filling significantly reduces wheat productivity by accelerating senescence, disrupting photosynthesis, and inducing oxidative damage. In the North-Eastern Himalayan state of Manipur, rising February–March temperatures have intensified late-season heat episodes, threatening wheat cultivation under the rice–wheat production system. The present investigation evaluated the role of antioxidant defense systems in maintaining yield stability of wheat (Triticum aestivum L.) under terminal heat stress conditions in valley and foothill agro-climatic zones of Manipur. Field experiments were conducted using ten wheat genotypes under normal and delayed sowing conditions to impose heat stress (≥30–35°C during anthesis). Biochemical parameters including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), malondialdehyde (MDA), and proline were quantified alongside physiological traits and yield components. Results indicated significant genotype × environment interactions (p < 0.01). Heat-tolerant genotypes maintained 25–40% higher antioxidant enzyme activity and exhibited 30–45% lower lipid peroxidation compared to susceptible lines. Grain yield reduction under late sowing ranged from 12% in tolerant genotypes to 38% in susceptible ones. Strong negative correlations were observed between MDA and grain yield (r = –0.78), while SOD and CAT activities were positively correlated with yield stability (r = 0.71–0.76). The findings demonstrate that robust antioxidant defense mechanisms are critical determinants of hyperthermal durability and yield resilience in Manipur’s wheat ecosystems.