Laser powder bed fusion (LPBF) was adopted to manufacture AlSi10Mg, and two post-processing schedules, T4 (510 °C/2 h + water quench) and T6 (T4 + 180 °C/6 h), were applied to elucidate how Si precipitation size controls ductility. The as-built alloy consisted of an α-Al matrix with a grid-like eutectic Si network and achieved UTS > 480 MPa but exhibited build-direction-dependent tensile anisotropy. Heat treatment promoted Si precipitation from the supersaturated α-Al matrix and transformed the eutectic network via fragmentation, spheroidization, and Ostwald ripening, leading to pronounced softening and improved elongation. After T4, the yield strength and UTS decreased by >50%, while elongation increased from 10.9% to 22.27%; T6 provided a slight strength recovery accompanied by a marginal ductility reduction. Mechanistically, a high number density of fine Si precipitates enhances dislocation storage and delays damage accumulation, whereas coarse, non-shearable Si particles intensify local strain gradients, facilitate void nucleation at the matrix/particle interface, and accelerate fracture. Overall, tailoring Si precipitation/coarsening offers an effective route to improve ductility and mitigate anisotropy in LPBF AlSi10Mg.