Poly(lactic acid) (PLA) is a biodegradable, bio-based polymer with promising applications in sustainable materials. However, its inherent brittleness, poor UV resistance, and limited thermal stability hinder broader use in advanced applications such as packaging and biomedical devices. Interface engineering through the incorporation of functional inorganic nanomaterials offers a potential route to overcome these limitations while maintaining the environmental advantages of PLA. In this study, we report on the design, synthesis, and characterization of PLA nanocomposites reinforced with surface-modified graphitic carbon nitride (g-C₃N₄, CN), a metal-free, 2D nanomaterial with unique optical and structural properties. CN was synthesized from melamine and subsequently functionalized using mild acid oxidation and silanization to improve dispersion and compatibility with the PLA matrix. These modifications introduced reactive –OH, –NH, and alkoxysilane groups on the CN surface, enabling hydrogen bonding and covalent interactions with PLA during melt blending. Mechanical testing showed that with only 1 wt% silanized CN, the elongation at break of PLA increased sevenfold, from 6% to 42%, without sacrificing tensile strength. Optical analysis demonstrated a dramatic reduction in UV transmittance below 400 nm, while visible-light transparency remained above 80%. These enhancements were attributed to improved interfacial adhesion and uniform nanofiller dispersion, as confirmed by means of FTIR, TEM, XRD, and XPS. This work demonstrates that interface engineering via surface-functionalized inorganic nanofillers can effectively enhance the mechanical flexibility, UV barrier, and thermal stability of PLA with minimal filler content. Our approach provides a scalable and environmentally conscious strategy for developing multifunctional PLA-based nanocomposites tailored for advanced material applications.