Abstract: To increase the processability and plasticity of the selective laser melting (SLM) fabricated Al–Mn–Mg–Er–Zr alloys, a novel TiB₂-modified Al–Mn–Mg–Er–Zr alloy with a mixture of Al–Mn–Mg–Er–Zr and nano-TiB₂ powders was fabricated by SLM. The processability, microstructure, and mechanical properties of the alloy were systematically investigated by density measurement, microstructure characterization, and mechanical properties testing. The alloys fabricated at 250 W displayed higher relative densities due to a uniformly smooth top surface and appropriate laser energy input. The maximum relative density value of the alloy reached (99.7 ± 0.1)%, demonstrating good processability. The alloy exhibited a duplex grain microstructure consisting of columnar regions primarily and equiaxed regions with TiB₂, Al₆Mn, and Al₃Er phases distributed along the grain boundaries. After directly aging treatment at a high temperature of 400°C, the strength of the SLM-fabricated TiB₂/Al–Mn–Mg–Er–Zr alloy increased due to the precipitation of the secondary Al₆Mn phases. The maximum yield strength and ultimate tensile strength of the aging alloy were measured to be (374 ± 1) and (512 ± 13) MPa, respectively. The SLM-fabricated TiB₂/Al–Mn–Mg–Er–Zr alloy demonstrates exceptional strength and thermal stability due to the synergistic effects of the inhibition of grain growth, the incorporation of TiB₂ nanoparticles, and the precipitation of secondary Al₆Mn nanoparticles.