Strawberries are sensitive to abiotic stresses such as salinity, high levels of electrical conductivity, and nutrient imbalances. The mutualistic endophytic fungus Piriformospora indica has significant potential to be used in improving crop production under adverse conditions, with a large host range. However, greenhouse production requires novel tactics to improve the efficiency of saline water irrigation in areas with limited freshwater resources. This study was conducted at the National Research and Development Center for Sustainable Agriculture (Estidamah), KSA, to investigate the impact of P. indica colonization on the growth, photosynthesis traits, productivity, and fruit quality of Fragraria × ananassa Duch cv. Festival strawberry grown in pots and irrigated with two electrical conductivity (EC) nutrient solutions of 1.5 and 3.0 dS/m. The results showed that higher-EC-nutrient solution clearly reduced growth and early yield and improved vitamin C, TSS, and anthocyanin of strawberry in comparison to low-EC-nutrient solution. On the other hand, P. indica colonization significantly increased plant height, shoot fresh weight, root length, and early yield of plants grown under high-EC-nutrient solution with no impact on fruit quality. Obviously, the symbiosis between strawberry roots and P. indica enhanced chlorophyll content, photosynthetic rate, stomatal conductance, and transpiration rate, as well as antioxidant activity such as proline, malondialdehyde, catalase, superoxide dismutase, and peroxidase under higher-EC-nutrient solution. Our study indicated that P. indica might be used as a sustainable tool for strawberry production in arid and semiarid zones, to mitigate the negative impacts of higher-EC-nutrient solution.