The increasing use of aluminium alloys in transportation industry, such as railways, shipbuilding and aeronautics, promotes the development of more efficient and reliable welding processes. Friction stir welding (FSW) is a prominent solid-state joining technology that arose as a possible reliable welding solution. Optimized process parameters are not regularly used in previous studies found in the literature, in particular T-joints, which difficult the process industrial application. This study is focused on the optimization of friction stir welded T-joints using the Taguchi method. Mechanical tests of 27 different welded joints were carried out, and results were analysed using ANOVA, mean effect and response surface methodology (RSM). The tool rotational speed was verified to be the most influent factor in the joint mechanical properties, and is strongly dependent on the shoulder/probe diameters ratio. It was also shown that using 1000 rpm, 3.90 mm of probe depth and shoulder/probe diameters ratio of 2.5 (shoulder diameter of 15 mm) it may be achieved improved joint strength. For the optimized parameters it was verified that the welding speed does not have a significant influence. Equations to predict the joints mechanical properties were also derived through multiple regression.