Applying some compounds such as salicylic acid and cerium oxide can be a valuable method to improve abiotic stress tolerance in plants. To assay the effects of salicylic acid nanoparticles (SA-NPs), cerium oxide (CeO2–NPs), and CeO2- SA nanocomposite on physiological, and biochemical properties and the percentage of essential oil of spearmint plant (Mentha spicata L.) under salinity stress, a factorial experiment was conducted based on a completely randomized design with 2 factors and 3 replications in greenhouse conditions. The frst factor was three levels of salinity stress (0, 50, and 100 mM), and the second factor included T2 (100 µM SA-NPs), T3 (50 mg L − 1 CeO2–NPs), T4 (25 mg L − 1 and 50 µM CeO2- SA nanocomposite) and T5 (50 mg L − 1 and 100 µM CeO2- SA nanocomposite). The activity of antioxidant enzymes such as superoxide dismutase, guaiacol peroxidase, ascorbate peroxidase, total soluble protein, malondialdehyde, hydrogen peroxide, proline, total ascorbate, the reduced and total AsA, DHA, AsA/DHA, elements (Na, K, Na /K, Zn, Cu, Mn, Fe) content, chlorophyll fluorescence (Fm, Fv, Fm/Fv, F0), percentage of essential oil, essential oil compounds and photosynthetic pigments content were evaluated. The results showed that with increasing salinity stress, the content of carbohydrates, protein, total antioxidants, carotenoids, chlorophyll a, chlorophyll b, and total chlorophyll decreased compared with the control. At the same time, cerium oxide-salicylic acid nanocomposite application improved protein, carbohydrate, phenolics, flavonoids, essential oil percentage, and total antioxidant capacity. However, proline, hydrogen peroxide, superoxide dismutase, guaiacol peroxidase, and ascorbate peroxidase increased signifcantly with enhancing salinity levels compared with the control. The application of cerium oxide-salicylic acid nanocomposite reduced the mentioned traits. The main essential oil constituents were pulegone, L-enthone, 1,8- cineole, a-terpinene, trans-cary
