<p>The protection and transmittance enhancement of infrared (IR) windows are critical for the performance and longevity of IR photoelectric systems. In this work, we report the fabrication and systematic study of yttrium oxynitride (Y–O–N) thin films as multifunctional coatings for zinc sulfide (ZnS) infrared windows. Y–O–N films were deposited via reactive sputtering, and their structural phase was precisely controlled by tuning the oxygen flow rate. The resulting films exhibit desirable mechanical properties, with hardness around 5.5 GPa and an elastic modulus in the range of 125–140 GPa. Optical gaps (~ 5.6&#xa0;eV) were found to be intermediate between those of yttrium oxide (Y₂O₃) and yttrium nitride (YN), indicating a tunable electronic structure. Importantly, by optimizing the film thickness based on measured refractive index (n) and extinction coefficient (k) data and guided by antireflection theory, we demonstrate that Y–O–N coatings can enhance IR transmittance of ZnS substrates up to 88% in the 2–12&#xa0;μm range—significantly higher than the ~ 70% of bare ZnS. This work introduces Y–O–N as a novel dual-function material for IR window applications, offering both mechanical protection and superior optical performance. Advanced characterization techniques were employed to reveal underlying structural and optical behaviors, shedding light on the correlation between deposition parameters, microstructure, and functional properties.</p>