<p>Mg-RE alloy is a difficult-to-weld metal, and its wire welding is rather challenging. In this work, we use the resistance upset butt welding to join Mg-RE alloy wire, and a high-quality welded joint is obtained, with the welded joint exhibiting a yield strength of 95.7% of base metal. This welded joint exhibits the narrow heat-affected zone (HAZ) and fusion zone (FZ), due to the low heat input and upset force. The FZ exhibits refined equiaxed grains, owing to the synergistic effect of the rapid solidification rate, upset force effect and Zr-refiner. In the FZ, Gd atoms are consumed to create coarse eutectic phase, while Er atoms are dissolved in the matrix. Therefore, high-strength welded joints are primarily attributed to the combined effects of grain refinement and Er-solute hardening. Compared to base metal, the reduced strength of the joint can be attributed to the weakened solute hardening effect as a result of the formation of eutectic phases depleting Gd solutes. Meanwhile, these eutectic phases are brittle, which deteriorates the ductility of the welded joint. This study provides an effective solution for welding Mg-RE alloy wires.</p> Graphical Abstract <p></p>