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Not AvailablePlant-derived protein hydrolysates represent new biostimulant products able to improve crop toler-ance to abiotic stresses. The aim of the study was to determine growth, root morphology, SPAD index,chlorophyll fluorescence, leaf mineral composition, and metabolic profiling of greenhouse lettuce eitheruntreated or treated (root or leaf-root application) with a plant-derived protein hydrolysate. For foliarapplication, plants were sprayed with a solution containing 2.5 ml L−1of biostimulant, whereas for rootapplication, 100 mL of solution with the same concentration was applied to the growing medium atweekly intervals. Lettuce plants were supplied with two nutrient solutions: non-salt control (1 mM NaCl)or 25 mM NaCl. Salt stress decreased shoot and root dry biomass, SPAD index, chlorophyll fluorescence,leaf mineral composition and increased foliar proline concentration. Root and leaf-root application ofthe biostimulant increased fresh yield, dry biomass and root dry weight of lettuce under salinity con-ditions. This was associated with an improvement of plant nitrogen metabolism and an increase of theFv/Fm-ratio efficiency in biostimulant-treated plants. Oxidative stress mitigation, increase in osmolytes,changes in sterols and terpenes composition, as well as the less expected increase in glucosinolates werealso observed in biostimulant-treated plants grown under saline conditions. The present study provesthat the application of plant-derived protein hydrolysate increases plant performance when plants aregrown under salinity conditions. The most favorable metabolic profile was obtained when biostimulantwas applied to both roots and leavesNot Availabl