The objective of this study is to provide a better knowledge of the relationships between microstructural evolution and both mechanical and corrosion properties of a friction stir welded Al-Mg-Sc alloy. Microstructures were studied by optical microscopy, transmission electron microscopy, electron backscattered diffraction and microhardness measurements. Tensile testing was carried out in order to determine the global behaviour of the joint as well as the local behaviour of the main zones of the joint. The weld nugget appears to be the softest zone of the weld. Its undermatching is analyzed with reference to the various hardening contributions of relevance in Al-Mg-Sc alloys. It is tentatively concluded that recrystallization which occurred in the nugget at the expense of the fine unrecrystallized structure of the base metal is primarily responsible for the loss of strength of the joint. Accelerated corrosion tests did not show any evidence of susceptibility to intergranular attack.