A novel in-situ-synthesis strategy via one-pot thermal decomposition of a single-source precursor was developed to fabricate silver sulfide (Ag₂S) nanoparticles with monodispersity and narrow size distribution in the polyamide-imide (PAI) nanocomposite coatings. The crystalline phases, size, morphologies, and growth mechanism of the in-situ-synthesized Ag₂S nanoparticles were characterized by X-ray diffraction, ultrahigh-resolution field emission scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The enhancement effect of such in-situ-synthesized Ag₂S nanoparticles on the microhardness and tribological behaviors of the PAI nanocomposite coatings were also investigated and compared with mechanically mixed Ag₂S particles. Results show that the growth of Ag₂S particles is greatly limited during the in-situ preparation process. The Ag₂S nanoparticles with small size ranging from 100 to 130 nm are purely formed and exhibit monodispersibility in the PAI nanocomposite coatings. Besides, the appropriate Ag₂S nanoparticles greatly promote the microhardness and antiwear performance of the PAI nanocomposite coatings, especially when their incorporation content reaches 5.0 wt%. The enhancement effects of these in-situ-synthesized Ag₂S nanoparticles on microhardness and wear resistance of PAI nanocomposite coatings are superior to that of the mechanically mixed Ag₂S particles. This is attributed to the improvement of cohesion strength and the load-carrying capacity of the PAI nanocomposite coatings enhanced by in-situ-synthesized Ag₂S nanoparticles.