<p>Si-containing Mg alloys solidified at conventional rates often contain coarse and sharp Mg₂Si phases, which can result in inferior material properties. In this study, Mg-6wt.% Si (Mg-6Si) alloy was prepared by wire arc additive manufacturing (WAAM), employing the gas tungsten arc welding technique with rapid cooling. The microstructures and mechanical properties of the WAAM alloy were investigated and compared with those of the as-cast samples produced using a metal mold. The results indicate that the WAAM Mg-6Si is harder and stronger than the as-cast samples. The microhardness of the WAAM Mg-6Si increases by 36.6% in comparison to that of as-cast Mg-6Si alloy. Furthermore, the average tensile strengths at room temperature and 150 °C increases by 63.4% and 21.3%, respectively. WAAM refines both the Mg₂Si phase and the overall grains, resulting in a homogeneous morphology and improved mechanical properties. The granular Mg₂Si phase, characterized by fine particles with a diffused distribution, shows a significant increase in concentration. The acicular Mg₂Si phase is distributed along the grain boundaries, and its concentration significantly decreases. The average grain size of the Mg₂Si phase is about 9.20 µm, about 5 times smaller. The refinement and distribution of the granular Mg₂Si phase, as well as the reduction in the amount of needle-like Mg₂Si particles, are the key factors for improving the mechanical properties of WAAM Mg-6Si alloy.</p>