The microstructure and the mechanical properties of a 6056 aluminium alloy Friction Stir-Welded (FSW) joint were investigated in the present study. The structure was analysed using light, scanning and transmission electron microscopy. The change in microstructure across the welded joint was found to correspond to significant variation in hardness. As in most FSW joints, the structure was characterised by the presence of a region of severely deformed grains in proximity of the weld nugget, i.e. of a region of fine recrystallised grains. Tensile tests showed that the joint material exhibited a rupture strength similar to the parent material, even though the former was significantly less ductile. This difference resulted in a reduction in ductility of the welded sheets. A T6 treatment increased tensile strength, but further reduced joint ductility. Nevertheless, the strength of the welded sheet was found to be very close (80-90%) to that of the base alloy.