In this study, AA5052 joints are fabricated by underwater friction stir welding and the process parameters are optimized for maximum UTS value by utilizing a developed mathematical model. The experiments are conducted by using Taguchi's L9 orthogonal array, and polynomial regression analysis is applied to generate the model. Statistical tools such as analysis of variance (ANOVA), coefficient of determination is applied to check the adequacy of the developed model at 95 confidence level. Type of welding tools is identified as the most influencing factor on deciding the mechanical properties of the joint, followed by tool rotational speed and tool welding speed. The optimum process parameters are identified by the Taguchi parametric design method. The results indicated that the optimum process parameters combinations for better mechanical properties is attained at tool rotational speed of 1500 rpm and tool welding speed of 100 mm/min, using taper threaded cylindrical tool. A maximum UTS value of 225.48 MPa is obtained and it is verified by confirmation test. Copyright © 2018 ASME