Abstract: Strong and ductile Al alloys and their suitable design strategy have long been desired in selective laser melting. This work reports a non-equilibrium partitioning model and the correspondingly designed Al–7.5Mg–0.5Sc–0.3Zr–0.6Si alloy. This model effectively quantifies the influence of Mg and Si on hot cracking in aluminum alloy by considering the non-equilibrium partitioning under high cooling rates in selective laser melting. The designed Al–7.5Mg–0.5Sc–0.3Zr–0.6Si alloy exhibits no hot cracks and achieves a remarkably enhanced strength–ductility synergy (a yield strength of (412 ± 8) MPa and a uniform elongation of (15.6 ± 0.6)%), superior to previously reported Al–Mg–Sc–Zr and Al–Mn. A tensile cracking model is proposed to explore the origin of the improved ductility. Both the non-equilibrium partitioning model and the novel Al–7.5Mg–0.5Sc–0.3Zr–0.6Si alloy offers a promising opportunity for producing highly reliable aluminum parts through selective laser melting.