Barley cultivation faces challenges from changing climate conditions, including the increasing threat of drought. This study explored the potential of a fertilizer derived from Hermetia illucens L. frass to enhance the development and health of spring barley under optimal and drought conditions. The experiment, conducted in a controlled greenhouse environment, employed various fertilization treatments, including cattle manure and two doses of H. illucens L. frass-based fertilizer. Comprehensive assessments were made through visual observations and physiological measurements, including chlorophyll fluorescence, leaf gas exchange, and CO2 exchange between the soil and the atmosphere. The results demonstrated that the application of H. illucens L. frass-based fertilizer significantly improved barley vigor and health compared to the control and cattle manure treatments, especially under drought stress. Physiological measurements revealed positive effects on chlorophyll fluorescence parameters, indicating enhanced photosynthetic efficiency. Leaf gas exchange parameters also reflected improved photosynthetic activity, with the H. illucens L. frass-treated plants outperforming others. This study provides valuable insights into the potential of insect-derived fertilizers, particularly H. illucens L. frass, as a sustainable and effective way to enhance crop resilience to drought. As climate change continues to pose challenges to agriculture, incorporating such novel fertilizers may offer a promising avenue for sustainable crop production.