Abstract: Cu-based alloys can be used as a selective laser melting (SLM) material for advanced engineering applications, such as aerospace, 5G mobile networks, and high-speed transportation. The mechanical properties and solidification microstructures of Cu alloys prepared using the casting technique differ from those prepared using the SLM technique, and SLM-built alloys can involve more complex microstructures and phase transformations developed in micromolten pools produced by high-power laser beams. However, nonequilibrium solidification microstructures and mechanical properties of SLM-built Cu–Sn alloys have seldom been studied in the literature. In this work, the Cu–5%Sn alloy was investigated using the SLM technique, along with cast Cu–Sn alloys for comparison. The high quality Cu-based alloy samples were fabricated using the SLM technique, with optimized processing parameters of 160 W laser power, 300 mm·s⁻¹ scanning speed, and 0.07 mm line spacing. The samples exhibit a relative density of 99.2%, and virtually no pores and spheroidizing phenomena or warping defects were observed. The microstructural analysis of SLM-built Cu–5% Sn alloy reveals a nonequilibrium solidification feature under high cooling rates and rapid alternative thermal conditions during the SLM fabrication process, in which the α-Cu(Sn) solid solution is the major phase along with γ and δ phases. Columnar grains and reticular microstructures dominate the solidified SLM-built alloy, while segregated Sn appears in the boundaries of all levels within the alloys. The Sn-rich nanoparticles with super-lattice structures precipitates along the grain boundaries and inside the grains. With the combined effects of grain fining, super-lattice-structured nanoparticles precipitation, solid solution, and thermal residual stress, the SLM-built Cu–5%Sn alloy shows significantly enhanced mechanical properties, such as HV 133.83 Vickers hardness, 326 MPa yield strength, 387 MPa tensile strength, and 22.7% fracture extension. Such scientific information is very useful for improving the alloy composition design and optimizing the SLM processing parameters.