<p>Maraging stainless steel (MSS) is widely employed in aerospace and pressure vessels due to its high strength, high toughness and good corrosion resistance. Microstructure design and combined deoxidation are important strategies for developing high-performance alloy steel. However, the evolution behavior and characteristics of non-metallic inclusions and their effects on the mechanical properties of additively manufactured MSS have not been conclusive for a long time. This work investigated the microstructure, inclusion characteristics, and mechanical properties of laser-directed energy deposited (LDED-ed) MSS from three kinds of raw materials with different deoxidizer contents. The results indicated that the microstructure of As-deposited (AD) MSS specimens primarily consisted of martensite with a minor fraction of retained austenite. After heat treatment (HT), the MSS specimens became fine and equiaxed grains with fine martensite packets/blocks, some reverted austenite, and dispersive Fe₂Mo nano-size particles. The HT MSS specimens exhibited an increased strength to ~ 1200 MPa, primarily due to the synergistic effects of Hall–Petch strengthening and precipitation strengthening. The inclusions in the AD MSS specimen were oxides, while those in the HT MSS specimen were composite inclusions, which consisted of oxide, TiN, and MnS. After heat treatment, a slight coarsening of the inclusions was observed; however, their number density remained constant. Compared to the HT counterpart specimen (MSS-I), both the HT MSS-LZr specimen with Zr content of 0.003 wt pct and the HT MSS-HZr specimen with Zr content of 0.026 wt pct by Ti–Al–Zr combined deoxidation had a higher impact toughness, due to the reduced amount of inclusions. The findings elucidate the critical role of combined deoxidation and heat treatment in tailoring microstructure and enhancing the mechanical properties of LDED-ed MSS.</p>