The current investigation was aimed to investigate the influences of different irrigation regimes, bio-stimulants and their interaction on onion yield and yield components, to improve plant growth under water stress, to reduce the negative effect of water stress. A split-plot desing was used, which the main plot was irrigation treatments (100% of crop evapotranspiration, 80% ETc and 60% ETc), while the bio-stimulants (control, yeast extract, potassium humate and yeast extract plus potassium humate) were assigned in sub-plot. The results showed that soil application of the biostimulants reduced the negative impact of water deficit compared to control. Yeast extract followed by yeast extract plus potassium humate recorded the highest plant height, diameter of neck and bulb, shoots fresh weight, fresh weight of bulb and total yield of onion during both seasons. With decreasing the required amount of water (at 80% ETc), the reduction percentage was only 5.18% and 4.65% when plants treated with yeast extract compared to 100% ETc in the 1st and 2nd seasons, respectively. However, yeast extract at 80% ETc was higher in the total yield than untreated plants under 100% ETc irrigation regime by 16.37% and 16.03% during the first and second seasons. Moreover, the highest irrigation water use efficiency was recorded at an irrigation regime 80% ETc and yeast extract with no significant difference with yeast extract plus potassium humate. From obtained results, yeast extract and or yeast extract plus potassium humate could play an important role in increasing plant stress tolerance to water deficit irrigation.