Friction Stir Welding (FSW) of titanium sheets was carried out under air and water environment, and the tensile properties of the joints made were measured. The tool rotational speed and tool traversing speed, which signi cantly in uence the tensile properties of the welded joints, were considered as input process parameters. This work deals with the application of the analytic hierarchy process to calculate the weights of the relative importance of the output responses using a pairwise comparison of responses and checks for the consistency and acceptability of the assumed comparison. Also, the Vlsekriterijumska Optimizacija I Kompromisno Resenje (VIKOR) optimization technique, a multi-response multi-criterion method, was used to determine the optimum process parameters. From the VIKOR optimization method, it is observed that the higher tool rotational speed and lower tool traversing speed are the optimum process parameters in both conventional and underwater FSW. The results from the experimental measurements and the study of microstructure support the results obtained from the VIKOR optimization method.