An Al–7.3 mass%Mg-based alloy with high strength and good hot rollability was developed by addition of grain boundary strengthening element. We predicted that Fe is the most effective element for grain boundary strengthening at 773 K from first-principles calculations. Then, an Al–7.3%Mg–62 ppmFe alloy and Al–7.3%Mg–124 ppmFe alloy were developed. Grain boundary analysis by HAADF-STEM revealed that added Fe segregated at grain boundaries in the Al–111 ppmFe alloy. The Al–7.3%Mg–62 ppmFe alloy and Al–7.3%Mg–124 ppmFe alloy exhibited good workability without crack by intergranular fracture, whereas an Al–7.3%Mg alloy exhibited poor workability with crack by intergranular fracture at 298–623 K. Also, the Al–7.3%Mg–124 ppmFe alloy showed the best workability, and when the Fe concentration was below the solid solubility limit, the workability improved as the amount of Fe added increased. We succeeded in developing a high strength Al–7.3%Mg–124 ppmFe alloy with good workability.