Abstarct: The use of natural compounds to enhance plant and mitigate the effects of water deficit stress rather than chemical compounds has attracted researchers' attention. Bio stimulants are low molecular weight molecules that can play a defense role in plants and are one of the most important ways to increase the production of secondary mextabolites in the plant that are capable of inducing physiological changes in the plant and have are of biological and non-biological origin.Yeast extract (Saccharomyces cerevisiae)is a biological stimulant due to its variety of minerals (thiamine, riboflavin, niacin, pyridoxine), vitamins (B1, B2, B3, B6 and B12), auxin, cytokinin, enzymes, amino acids, carbohydrates, lipids Nucleic acids and nutrients have a positive role in reducing the harmful effects of drought stress. For this purpose, in order to investigate the effect of foliar application of yeast extract and water deficit stress on physiological and morphological characteristics of cowpea, an experimental factorial experiment was conducted in a completely randomized complete block design with three replications. The Irrigation was applied in plots every seven days and in plots under water deficit stress, complete irrigation was stopped at 50% flowering and pod formation for ten days. Foliar application of yeast extract (2, 4, 6 g / L and control) was repeated 2 times with 7days interval and the first time was 30 days after planting. The results showed that chlorophyll b, antioxidant enzymes, reducing and non-reducing sugars were significantly increased at 6 g / L of yeast extract, and grain yield was also highest at this concentration. Plant height, grain protein and nitrogen increased significantly under 4 g / l treatment. The fresh and dry weight of root nodules with the concentration of 2 g / l was the highest. Concentrations of 2 to 6 g / l yeast increased proline, chlorophyll a, anthocyanin, flavonoid, starch compared to control. Interaction of yeast extract and water deficit stress on antioxidant enzymes, photosynthetic pigments, nitrogen-phosphorus and seed protein, number and weight of root nodules, grain yield, and membrane stability were significant. Therefore, yeast extract can increase the defensive response to environmental stresses as an ecological strategy.