Abstract: Ceramic particle reinforcement has been widely used in hardening metallic materials with relatively low strength. In this work, this strategy was applied in equiatomic FeCrNi medium-entropy alloy (MEA) produced using a conventional laser powder bed fusion (LPBF) method, under different volumetric energy density (VED) values. Leveraging the characteristics of the LPBF process, the incorporation of micron-sized TiB2 ceramic powders enabled the in situ formation of nanoscale TiB2 particles, and a portion of these particles were melted during printing and subsequently oxidized to nano-sized TiO2 particles. The as-printed samples exhibited distinct macroscopic printing defects, including rough surfaces and pores, along with cellular microstructures where elemental segregation of primary Cr occurs at cell boundaries. With increasing VED, the morphology of the σ phase, which was motivated by local Cr segregation, was found to evolve from an interconnected skeletal structure to an accumulation of numerous short-rod-shaped particles. These particles effectively pinned a high density of dislocations, thereby hindering dislocation motion. By modulating the VED, we tailored the morphology and distribution of the σ phase into an effective strengthening constituent, thereby achieving enhanced yield strength. The yield strength of the alloy processed with the highest VED of 116.9 J/mm3 achieved the maximum value of ~960 MPa among all. This represents a ~28.9% increase, equivalent to approximately 215 MPa, compared to TiB2-free samples produced via LPBF. Microstructural analysis and theoretical calculations evidence the synergistic strengthening mechanisms of the TiB2-modified FeCrNi MEA. dislocation interaction with various types of small particles, accompanied by additional contributions from grain refinement and load transfer effects, collectively forming a multi-dimensional strengthening mechanism system.