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Zinc oxide nanoparticles (ZnO-NPs) have been proven to helpfully improve plant tolerance to several abiotic stresses. However, no information has been reported concerning the role of ZnO-NPs on pepper plants under salinity stress. Hence, this research aimed to evaluate the growth and physiological responses of pepper (<i>Capsicum annum</i> L.) plants to ZnO-NP foliar application under salinity. Plants were subjected to 0 (control), 25 (S1), 50 (S2), and 75 mM (S3) NaCl salinity with a foliar spray of 0, 1000, and 2000 ppm ZnO-NPs. Significant reductions were recorded in the chlorophyll index (SPAD) and chlorophyll fluorescence parameters, and in the activity and/or ratios of reduced ascorbate (AsA), reduced ascorbate/dehydroascorbic acid (AsA/DHA), reduced glutathione (GSH), reduced glutathione/oxidized glutathione (GSH/GSSG), and K⁺ content. There was a significant increase in proline content, electrolyte leakage (EL), H₂O₂ content, guaiacol peroxidase (GPX), ascorbate peroxidase (APX), superoxide dismutase (SOD), glutathione reductase (GR), dehydroascorbic acid (DHA), and oxidized glutathione (GSSG) activities, and in Na⁺ content and Na⁺/K⁺ ratio. Foliar treatments improved the salinity tolerance of the pepper plants by fortifying the antioxidant defense system, leaf fluorescence parameters, K⁺, and proline content, and in contrast, by decreasing the EL, Na⁺, and H₂O₂ levels. ZnO-NP foliar treatment efficiently improved the pepper plants’ physiological responses under salinity. Considering the overall results, 1000 ppm of ZnO-NPs would be advisable for the amelioration of salinity depression and to promote growth potential. However, at higher levels, the nanoparticle showed toxicity symptoms that limited its reliable applications.