In order to study the effect of physiological and biochemical responses of tomato cv. Rio Grande to foliar spray of Megafol under water deficit stress, an experiment was carried out in a split plot based on randomized complete block design with three replicates in Research Filed of university of Zanjan as during 2015. Treatments consisted arrangement of three irrigation levels (starting irrigation at 100, 75 and 50% ETc) and 4 levels of Megafol (0, 0.1, 0.2 and 0.3%). The results showed that water deficit stress significantly decreased relative water content, chlorophyll content and stomatal conductivity, but increased proline content, electrolyte leakage and catalase (CAT) and peroxidase (POD) enzymes activity. Foliar application of Megafol increased chlorophyll and proline content, CAT and POD enzymes activity and stomatal conductivity under water deficit stress and normal irrigation coditions, and decreased electrolyte leakage, that the 0.2% Megafol treatment had the highest effect. The maximum CAT enzyme activity (0.087 µmol H₂O₂ g^-1 FW min^-1) was achieved in plants sprayed with Megafol 0.2% under defici irrigation 50% ETc. According to the results, foliar application of biostimulants can be proposed on tomato under normal and deficit irrigation.