In arid areas, low soil fertility, an imbalance of nutrients, and the inability of crops to absorb some nutrients are among the main constraints on crop production. The use of biological fertilizers and biostimulants can be a suitable solution. A 2-year field experiment was conducted as a factorial experiment based on a randomized complete block design with three replications to study the response of castor bean to arbuscular mycorrhizal fungi (AMF) and amino acid biostimulant (AAB) (foliar, soil, and combined application methods). Findings showed that plants inoculated with AMF had better performance: the biological and seed yields were increased by an average of 20.9 and 26.4% over the 2 years of the experiment. The yield components showed a 14.0 to 18.6% increase, and the water productivity (WP) was improved by 13.9%. The seed oil content was increased by 5.1%: linoleic acid, an oil fatty acid, showed the highest response, with an increase of 10.0%. Among the AAB treatments, the combined application method (CAM) brought the best results. The highest increases in biological and seed yields (68.4 and 63.2%, respectively) were obtained from the CAM treatment. The WP was improved by 67.8% and the seed oil content showed an increase of 9.4%. Among the fatty acids, the highest increase (24.4%) belonged to linoleic acid. The results show a positive and significant response of castor beans to biostimulants, which indicates that replacing chemicals with biological fertilizers could be a promising approach to the cultivation of castor beans in arid climates.