Valorization of agricultural residues into biochar for soil applications offers dual benefits of waste management and sustainable agriculture. However, the mechanisms governing sandy soil and lettuce response to biochar under deficit irrigation are not well understood. This study evaluated the effects of biochar types on sandy soil physiochemical properties and lettuce (Lactuca sativa L.) yield at different irrigation levels. A field experiment was performed using a randomized complete block design with four treatments (soil only, cotton stalk biochar, cashew nutshells biochar, and a mix of cotton stalks+ cashew nutshells biochar) and three irrigation regimes (100%, 80, and 60% of crop water requirements ETc) in Ouagadougou, Burkina Faso. The results showed that biochar-amended soils had consistently higher water retention and macronutrients, resulting in higher fresh, marketable lettuce yields under deficit irrigation compared to untreated soils. Compared to other treatments, a mix of cotton-stalk and cashew-nutshell biochar produced the highest yield (18.1 tons/ha) under moderate irrigation (80% ETc). Achieving optimal yields with 20% less irrigation water indicates biochar’s water-saving potential in climate-resilient vegetable farming. These findings underscore the potential of combining deficit irrigation and biochar for sustainable vegetable production to mitigate food security in water-scarce regions.