Cowpea provides the cheapest source of protein with an average range of protein content of 23–30%. However, cowpea growth, development, and yield are greatly affected by drought during flowering and pod filling in the sub-Sahelian areas. e best way to cope with this situation is to develop drought-tolerant cowpea varieties.e objective of this study was therefore to evaluate cowpea lines developed through mutagenesis using gamma radiation to assess their reaction under optimal and water-stressed conditions.e response of ten mutants-irradiated Moussa local was then evaluated in pots arranged in a split plot design in a screen house. Two conditions were applied with optimum and water-stressed conditions. e stress was applied for two weeks at f lowering. Two cowpea varieties Gorom local (drought-tolerant) and Moussa local (susceptible) nonirradiated were used as checks.ereafter, field trials under two different sowing dates were conducted to identify the best tolerant mutant line(s) using agromorphological and tolerance indicators. e results indicated that mutant lines (MoussaM51-4P10 and MoussaM43-20P14) exhibited better stress tolerance and produced higher yield under water stress conditions. Stress Tolerance Index (STI) was better to select cowpea mutant tolerant with higher yielding under moderate stress (SI�35%). e study confirmed that water stress has a negative effect on cowpea seeds production and on leaf chlorophyll content. e high temperature during experiment increased water stress effect mainly on non-irradiated checks (Gorom local and Moussa local).