<p>Salinity stress is a major abiotic factor limiting crop productivity. This study investigated the efficacy of nano-silicon (NSi) and conventional silicon (Si) in mitigating salinity effects on cluster bean. Plants subjected to salinity stress (0, 6, and 12 dS m^− 1) were treated with foliar applications of NSi or Si (50, 100, 200 mg L^− 1). While both forms of silicon ameliorated salt-induced damage, NSi was significantly more effective. The 50 mg L^− 1 NSi treatment emerged as the optimal concentration, markedly enhancing antioxidant enzyme activities, reducing oxidative stress markers (MDA and H₂O₂), and restoring ion homeostasis by decreasing Na⁺ and increasing K⁺ accumulation. These physiological improvements translated into agronomic benefits, with a 2.8-fold increase in seed weight under high salinity. Crucially, NSi at 50 mg L^− 1 superiorly improved seed oil quality by significantly increasing the proportion of nutritionally valuable unsaturated fatty acids (C18:1, C18:2), a change strongly correlated with enhanced antioxidant capacity. The results demonstrate that NSi, particularly at 50 mg L^− 1, outperforms conventional Si by more effectively bolstering the plant’s antioxidant system and ion regulation, leading to significantly higher yield and better seed quality under salt stress. This positions NSi as a potent nano-biostimulant for sustainable crop production in saline conditions.</p>