Abstract: An Al-Mg-Sc-Zr thin-wall deposit was fabricated by coaxial laser wire additive manufacturing, and its microstructure, mechanical properties, and strengthening mechanisms in the as-deposited condition and after heat treatment at 325°C were investigated. The results showed that the deposit was continuously formed. The as-deposited microstructure consisted predominantly of equiaxed grains, with alternating fine-grained bands and coarse-grained regions along the build direction; the mean grain sizes of the fine-grained bands, coarse-grained regions, and overall region were 7.24, 24.13, and 8.41μm, respectively. BSE-EDS analysis revealed micrometre-scale Sc- and Zr-rich particles, with little change in the overall morphology of the coarse particles after heat treatment. TEM observation showed 0.1–0.5μm particles and a heterogeneous dislocation structure in the matrix. Heat treatment increased the average microhardness from approximately 84HV to 102–104 HV. After holding at 325°C for 3h, the ultimate tensile strengths in the H and V directions reached 385.68 and 380.64MPa, respectively, with corresponding fracture strains of 18.45% and 15.94%. When the holding time was extended to 6h, the yield strength remained essentially stable, whereas the ultimate tensile strength and ductility decreased. Hall–Petch estimates indicated that the grain-boundary strengthening contributions of the overall region and the local fine-grained band were 58.6 and 63.2MPa, respectively, while the average net increase in yield strength induced by heat treatment was 37.8–39.5MPa. All specimens exhibited ductile fracture governed by microvoid coalescence; more rapid void linkage in the V direction resulted in a lower fracture strain.