<p>This study aims at the development and characterization of polylactic acid (PLA)—silicon nitride (Si₃N₄) composites for fused deposition modelling (FDM) 3D printing. Polymer ceramic composite filaments with varying Si₃N₄ concentrations (3%, 5%, and 7% by weight) were fabricated and extensively characterized. A quasi-isotropic layup was chosen as the raster pattern in the FDM process to rule out possibilities of anisotropy in mechanical properties. A detailed thermal and mechanical characterization, and morphological analysis were conducted on both the composite filaments and 3D-printed specimens. The PLA/Si₃N₄ composition with a weight ratio of 95:05 was found to be the optimal, improving the thermal and mechanical characteristics. The optimal weight percentage of the PLA–Si₃N₄ showed elevation of 9.4% and 11.1% in the glass transition temperature and in the melting point temperature, respectively when compared to that of pure PLA. The detailed mechanical characterization reveals that reinforcing PLA with Si₃N₄ in the optimum weight percentage of 95:05 showed significant influence such as enhancement of 19.04% in tensile strength, 19.6% in flexural strength, 25.5% in compressive strength, and 11.7% in impact strength when compared to pure PLA. Analysis using scanning electron microscopy (SEM) showed better uniformity and lower porosity when 5% of Si₃N₄ was added, which resulted in stronger mechanical characteristics. Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) confirmed the retention of α-phase Si₃N₄ structure within the polymer matrix. The increase in Si₃N₄ content led to a higher melt flow index, which suggested better processability for 3D printing. This comprehensive study provides valuable insights into the structure–property relationships of PLA–Si₃N₄ composites, paving the way for the development of high-performance, biodegradable materials for additive manufacturing applications in various industries.</p>