Abstract: Many studies have investigated the selective laser melting (SLM) of AlSi10Mg and AlSi7Mg alloys, but there are still lack of researches focused on Al–Si–Mg alloys specifically tailored for SLM. In this work, a novel high Mg-content AlSi8Mg3 alloy was specifically designed for SLM. The results showed that this new alloy exhibited excellent SLM processability with a lowest porosity of 0.07%. Massive lattice distortion led to a high Vickers hardness in samples fabricated at a high laser power due to the precipitation of Mg₂Si nanoparticles from the α-Al matrix induced by high-intensity intrinsic heat treatment during SLM. The maximum microhardness and compressive yield strength of the alloy reached HV (211 ± 4) and (526 ± 12) MPa, respectively. After aging treatment at 150°C, the maximum microhardness and compressive yield strength of the samples were further improved to HV (221 ± 4) and (577 ± 5) MPa, respectively. These values are higher than those of most known aluminum alloys fabricated by SLM. This paper provides a new idea for optimizing the mechanical properties of Al–Si–Mg alloys fabricated using SLM.