Magnesium and its alloys have the lowest density among structural metallic materials; thus, this light-weight metal has great potential for reducing the weight, for example, of vehicles and trains. However, due to its crystal structure, deformability is poor; in particular, under compressive stress. In this study, we modified magnesium with bismuth as an alloying element, which has the characteristics of being likely to form precipitates instead of grain boundary segregation. The Mg-Bi binary alloy showed excellent deformability and high absorption of energy in high-strain rate regimes at room temperature via contribution of grain boundary sliding. These properties, which are closely comparable to those of conventional middle-strength aluminum alloys (Al-Mg and Al-Mg-Si series alloys), have never been observed before in magnesium alloys. The development of such properties opens the door for not only academic but also industrial research in magnesium.