A novel Al−Mg−Er−Zr alloy was welded by highly efficient tandem GMAW, and the effect of Er on the porosity, microstructure and mechanical properties of the weld joints were investigated by comparing them with Al5083 alloy weld joints. The results showed that compared to the welds without Er addition, the porosity decreased from 0.78% to 0.38%, which was attributed to the ability of Er to reduce the content of free hydrogen in the welds. Furthermore, the grain size near the fusion boundary and weld metal (WM) was significantly refined with the addition of Er. Grain refinement near the fusion boundary was attributed to the heterogeneous nucleation induced by Al₃(Er,Zr) from base metal, while grain refinement in WM was caused by the hindering effect of Al₃Er on grain boundary migration. Additionally, the ultimate tensile strength of the weld joints increased from 272.44 to 338.74 MPa with Er addition. The precipitation strengthening induced by Al₃Er contributed to an increase by 20% in strength.