AbstractAn AlSi3.5Mg2.5 (wt%) alloy with excellent mechanical properties was produced via laser powder bed fusion in this study. The yield strength, tensile strength, and elongation of this as-built AlSi3.5Mg2.5 alloy reach about 406 MPa, 501 MPa, and 8.6%, respectively. These properties are dramatically superior to the current additively manufactured Al-Si-Mg alloys. A direct-aging treatment at 170°C for one hour increases the yield strength and ductility further to about 417 MPa and 11.0%, respectively, with the tensile strength remaining the same level. The microstructures and strengthening mechanisms of the as-built and direct-aged samples were investigated systematically. The underlying micromechanical mechanisms of the as-built and direct-aged samples were examined based on a combination of in-situ synchrotron X-ray diffraction and three-dimensional crystal plasticity modeling. The as-built AlSi3.5Mg2.5 alloy possesses a fine microstructure, including fine grains and nano-sized Mg2Si and Si precipitates. After direct-aging treatment, additional Mg2Si and Si precipitate out. Besides, element diffusion upon aging treatment causes migration of cell boundaries and relaxation of residual stress. The direct-aging treatment leads to an increased Orowan strengthening, dislocation strengthening, and load-bearing strengthening effects. Moreover, the variations of microstructure and residual stress after the aging treatment change the dislocation behavior and increase the dislocation storage capacity, causing an increased ductility. Nevertheless, the aging treatment does not alert the type of damage and fracture. This study provides valuable insights to tailor the microstructure and mechanical properties of additively manufactured Al-Si-Mg alloys.