Abstract: Owing to their excellent mechanical performance, triply periodic minimal surface (TPMS) structures have attracted increasing attention in engineering applications. The mechanical behavior of additively manufactured metallic TPMS lattices is governed by both macroscopic geometric topology and process-induced microstructural features. However, the relative contributions of these factors have not yet been sufficiently quantified. In this study, Diamond and Primitive TPMS lattice structures of 15-5PH stainless steel with different unit-cell sizes were fabricated by laser powder bed fusion. Experimental characterization was combined with crystal plasticity finite element (CPFE) simulations. This approach was used to investigate the structural characteristics, microstructural features, and mechanical properties of the TPMS lattices. Particular emphasis was placed on quantifying the respective contributions of macroscopic geometry and microstructure to the overall mechanical response. The results show that the fabricated TPMS lattices exhibit high strength, reaching a maximum value of 2487 MPa. They also demonstrate good ductility, with an elongation of 23.4%, and excellent resistance to crack propagation. Compared with the Primitive structures, the Diamond structures exhibit finer grains, higher dislocation densities, and diagonally intersecting stress–strain fields, whereas the Primitive structures are characterized by horizontally intersecting stress–strain fields. The CPFE simulations further reveal that macroscopic geometry plays a dominant role in determining the mechanical properties, contributing 52.3%–88.3%, whereas microstructural factors, including grain size and dislocation density, account for 11.7%–47.7%. These findings elucidate the synergistic effects of macroscopic geometry and microstructure on the mechanical behavior of metallic TPMS lattices and provide theoretical guidance for the design of high-performance lattice structures.