<p>Foliar treatment with citrates, particularly in the form of nanoparticles, is crucial for mitigating damage and enhancing plant physiology and metabolism during stress. This study is the first to examine the effects of foliar spraying with nanoparticle-based citric acid (n-CA) and conventional citric acid (c-CA) during the 2023 and 2024 seasons. It examined their impact on physiology, osmo-stress relievers, total antioxidant activity, enzyme gene expression, and fruit quality in hot pepper plants grown under salt stress (ECe = 8.5 dS m^–1). The optimal n-CA and c-CA concentrations were 0.4 and 4.0 mM, respectively, with 0.4 mM n-CA significantly outperforming 4.0 mM c-CA. Applying n-CA or c-CA to salt-stressed plants improved plant physio-biochemical attributes, including photosynthesis (52 or 37%), leaf fitness (60 or 46%), gas exchange (77 or 59%), nutrient contents (25 or 16%), K⁺/Na⁺ ratio (67 or 34%), hormonal levels (26 or 17%), and osmo-stress reliever contents (52 or 37%). Additionally, n-CA or c-CA improved antioxidant levels (36 or 24%), antioxidant redox states (22 or 14%), enzyme activities (27 or 17%), total antioxidant activity (20 or 14%), and enzyme gene expressions (177 or 141%). In contrast, treatment with n-CA or c-CA reduced superoxide; O₂^•– and hydrogen peroxide; H₂O₂ (66 or 52%) and their associated damage; electrolyte leakage and malondialdehyde levels (69 or 53%). Plant growth (35% or 21%), production (192% or 76%), and fruit quality (24% or 14%) were increased with n-CA or c-CA treatment, respectively. These outcomes suggest the crucial role of nanoparticle-based citrates in enhancing plant defense mechanisms against salt stress effects, providing a practical agronomic strategy to improve crop resilience to saline conditions.</p>