The industrial application of magnesium alloys is limited by poor corrosion resistance, and microalloying is a crucial method for the cost-effective enhancement of this property. This study developed new magnesium alloys, Mg-0.5Zn-0.4Mn-0.4Sm-0.4Al (alloy A) and Mg-0.5Zn-0.4Mn-0.4Sm-1.2Al (alloy B), by microalloying techniques. The microstructures and corrosion resistance of these new alloys were investigated through scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), weight loss, and hydrogen evolution experiments. The results indicated that both alloys consisted of an α-Mg matrix, a granular Mg41 Sm5 phase, and a dendritic Al11 Sm3 phase. The Al11 Sm3 phase is enriched along the grain boundaries, whereas the Mg41 Sm5 phase is dispersed within the grains. Compared with alloy A, alloy B has a greater quantity of the Al11 Sm3 phase, a smaller grain size, and improved corrosion resistance.