Additive manufacturing of nickel-based superalloys such as VZh159 using selective laser melting (SLM) enables the fabrication of complex geometries for high-temperature applications, but often leads to anisotropic microstructures, residual stresses, and cracking – prompting interest in ceramic nanoparticle reinforcement. This study investigates the effects of titanium diboride (TiB2) nanoparticle additions (0.5 – 2.0 wt.%) on the microstructure, density, and mechanical properties of VZh159 nickel-based superalloy fabricated using SLM. TiB2 additions effectively refined the dendritic structure from ~1.65 μm to ~0.5 μm and increased microhardness from 290 HV to 349 HV, but drastically reduced ductility from 35.9% to 3 – 5% at room temperature. Post-process heat treatment (solution at 1,100°C + two-stage aging at 800°C/700°C) transformed the dendritic structure into equiaxed grains; however, composites with >0.5% TiB2 exhibited severe grain boundary embrittlement due to boride segregation. Optimal SLM parameters shifted toward higher energy densities (from 85 – 120 to 130 – 160 J/mm3) with increasing TiB2 content. The study reveals that in situ transformation of TiB2 into complex chromium boride phases during SLM, while beneficial for structure refinement, creates a brittle grain boundary network that is detrimental to ductility. These findings provide critical insights for the development of particle-reinforced superalloys for additive manufacturing.