Water jet peening is the unconventional surface treatment technique which is gaining importance in advanced manufacturing processes. In contrast to conventional peening operations, water jet peening acts as a simple process control, despite the presence of multiple variables in the process. This paper describes a pure water jet employed for impingement on the target surface of aluminium 6061-T6 alloy with a pure water jet cutting nozzle. In this study, experiments were conducted with different water jet pressure by varying the standoff distance, traverse speed and number of passes. Determination of the optimal water jet peening parameters for the aluminium alloy by the multi-response technique TOPSIS was another objective. The experimental results indicated improvement in the micro hardness and surface roughness of the peened surface over the base material. The compressive residual stress was measured for optimal setting conditions. No significant changes were observed in the water peened surface.