<p>Salinity stress severely limits the productivity and quality of fennel (<i>Foeniculum vulgare</i> L.) by disrupting physiological, biochemical, and metabolic processes. Despite the proven benefits of zinc oxide nanoparticles (ZnO NPs) and cold plasma (CP) in improving plant stress tolerance, their combined effects and underlying synergistic mechanisms in fennel have not yet been explored. This study aimed to evaluate whether CP seed priming with CP at 100&#xa0;s could enhance the efficacy of foliar application of ZnO NPs (100 and 200 mg L^− 1) in mitigating salinity-induced damage under greenhouse conditions. Mature plants were harvested after 140 days to determine shoot biomass, seed yield, essential oil (EO) content and composition, and fatty acid profile. Salinity (60 and 120 mM NaCl) significantly reduced plant growth and yield, while the combination of CP and ZnO NPs—especially at 200&#xa0;mg L⁻¹—significantly improved these traits. The CP + ZnO treatment enhanced shoot and seed yield by 29% and 27%, respectively, under high salinity. EO content peaked at moderate salinity, while severe salinity further decreased limonene (by 24%) and increased anethole (by 9%) relative to the control. Moreover, CP and ZnO NPs increased unsaturated fatty acids instead of saturates ones, thereby mitigating oxidative damage under stress. Principal component analysis distinguished CP and ZnO co-treatments as superior clusters associated with improved metabolic performance. This study fills an important research gap by demonstrating, for the first time, that CP seed priming enhances the effectiveness of ZnO NPs, resulting in improved fennel growth, resilience, and EO quality under salinity stress. These findings highlight a sustainable and practical strategy for cultivation in saline environments.</p> Graphical Abstract <p></p>