Drought stress significantly constrains broccoli (Brassica oleracea L.) productivity by impairing growth, photosynthesis, and yield. Seed priming with ascorbic acid (AsA) has shown promise in enhancing early seedling performance; however, its effects on head development and yield under water deficit remain limited. This greenhouse pot experiment evaluated four seed treatments: non-primed control, water-primed control, 1 mg L⁻¹ AsA, and 10 mg L⁻¹ AsA under two irrigation regimes: 100% and 50% field capacity. Growth, physiological traits, biochemical responses, and yield were assessed. AsA priming significantly (p < 0.05) enhanced plant height, net photosynthesis, and chlorophyll content under both water regimes. Under 100% FC, water priming significantly increased canopy length, whereas under 50% FC, only AsA priming produced a significant increase relative to the non-primed control (p < 0.05). Biochemical responses further showed that 10 mg L⁻¹ AsA significantly (p < 0.05) increased chlorophyll a and chlorophyll b under 50% FC compared with the non-primed control. Proline accumulation was reduced by 10 mg L⁻¹ AsA, but this reduction was significant (p < 0.05) only under 100% FC. Under 100% FC, 10 mg L⁻¹ AsA significantly (p < 0.05) increased total phenolic content compared with the non-primed control. Total flavonoid content was significantly (p < 0.05) increased by 1 and 10 mg L⁻¹ AsA compared with the control, while both water priming and AsA priming significantly (p < 0.05) increased carotenoid content and reduced H₂O₂ accumulation relative to the non-primed control, irrespective of watering regime. Total yield per plant, measured on a fresh weight basis, significantly (p < 0.05) increased with increasing AsA concentration, with 10 mg L⁻¹ AsA enhancing yield by 37.8% relative to the water-primed control and by 70.5% relative to the non-primed control, independent of water regime. Percentage dry weight was unaffected by AsA treatment. Overall, AsA seed priming potentially enhanced physiological resilience and fresh yield of broccoli under water-limited conditions, indicating its potential as a low-cost strategy for drought mitigation.