<p>Magnesium rare earth (Mg-RE) alloys generally exhibit insufficient strength and ductility because of their coarse grains, which restricts their engineering applications. Grain refinement can be effectively achieved by adding Al. This study investigates the effect of Al addition on grain refinement and the mechanical properties of a Mg-6Gd-3Y alloy. Mg-6Gd-3Y alloys containing different Al contents (0, 0.5, 1.0, 1.5, 2.0, and 3.0 wt.%) were prepared by gravity casting. The microstructural evolution and the orientation relationship between the Al₂RE phase and the α-Mg matrix were characterized by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that with increasing Al content, the grain size first increases, then decreases, and finally increases again. When the Al content is 1.5 wt.%, the grain size reaches a minimum of 38.2±2.7 µm. This grain refinement is attributed to the formation of the Al₂RE phase promoted by Al addition, which provides effective heterogeneous nucleation sites for α-Mg. Room-temperature tensile tests and fracture analysis indicate that the Mg-6Gd-3Y-1.5Al alloy exhibits an excellent combination of tensile strength, yield strength and ductility. Its tensile and yield strength are comparable to those of the Mg-6Gd-3Y-0.5Zr alloy, whereas its elongation is higher, mainly due to second-phase strengthening and grain refinement. Furthermore, the alloying reactions during the solidification of the multicomponent Mg–Gd–Y–Al alloy system were elucidated. These findings provide a new strategy for optimizing the performance of Mg-Gd-Y alloys.</p>