This is the first research to investigate the Underwater Friction Stir Welding of Pipes (UFSWoP). UFSWoP of Al 1050 pipes was carried out successfully by designing a new device that allows the underwater rotational motion of the pipe to be friction stir welded. An important consideration in the UFSWoP is the cooling effect on the surrounding water during the process. The water-cooling affects directly the mechanical properties of the produced joint. A study is carried out to conclude and predict the effects of the UFSWoP process parameters on the mechanical properties of the welded joint. A Box Behnken experimental design was employed with different levels of tool rotational speed, traverse speed and tool diameter. The corresponding response considered was the tensile strength of the produced joints. Finally, a hybrid Response Surface Methodology-Fuzzy based model was proposed and evaluated for predicting the considered response of the UFSWoP process. This model showed more accurate predicted results than the Artificial Neural Network based model.