High-strength stainless steel 15-5PH (EOS PH1) combines excellent mechanical properties with good corrosion resistance and is widely used in aeronautical and aerospace industries. The properties presented by this material and its high value-added applications make it an excellent choice for manufacturing using additive manufacturing processes, such as laser powder bed fusion (L-PBF). This work reports the microstructural characterization of 15-5PH stainless steel manufactured by L-PBF under the as-built condition and after heat treatment. Low porosity (<1%) and high hardness (420 HV0.1) were observed for the as-built samples. Two distinct areas within the melting pools were observed: a region with coarser martensite laths at the bottom of the melting pools and a thermally affected zone composed of fine martensite in the upper region of the melting pool with high hardness and a larger amount of retained austenite. The fraction of this austenite remained significantly high after aging at temperatures of up to 621°C, directly from the as-built condition, for up to 8 h. However, after solubilization at 1040°C, no austenite was found in the samples even after aging. This retained austenite is metastable and transforms into strain-induced martensite when cold-deformed by the transformation-induced plasticity effect (TRIP). Aging also promotes steel hardening owing to the precipitation of Cu-rich cores. SEM and TEM images showed the presence of nanometric inclusions of silicon oxide in samples from all conditions manufactured by additive manufacturing. The samples manufactured by L-PBF showed excellent mechanical properties, reaching up to 1.7 GPa of maximum stress after aging at 482°C for 4 h, which is much higher than that reported for the material obtained by the conventional processing route. In aging carried out at 621°C, the reversion of nickel-rich austenite, predicted by kinetic calculations, was found by spectrographic means in more refined regions and close to the copper-rich precipitates, which presented an oval shape. The combination of refined martensite laths, copper-rich precipitates, oxide nanoparticle dispersion, and strain-induced martensite makes 15-5PH stainless steel manufactured by L-PBF a promising material for structural engineering applications.