The growing demand for sustainable packaging, stricter regulations on non-biodegradable plastic waste, and increasing consumer awareness of environmental pollution are driving the development of water-soluble packaging materials. This study investigates the potential of lignin nanoparticles (LNPs) derived from spruce kraft lignin (SKL) and eucalyptus kraft lignin (EKL), as functional additives in polyvinyl alcohol (PVA)-based films to achieve an optimal balance between high transparency and effective UV protection. To improve LNP dispersion within the PVA matrix, hydrophobic domains were introduced into lignin via acetylation, as confirmed by ³¹P NMR spectroscopy. The morphology of the nanoparticles was analyzed using transmission electron microscopy (TEM). The resulting PVA–LNP nanocomposite films exhibited excellent transparency and outstanding UV-shielding capabilities. UV–Vis spectroscopy confirmed the UV-blocking performance of the films, revealing that EKL-derived nanoparticles (EKL-C1) enhanced UV absorption more than eightfold compared to neat PVA, while SKL-derived nanoparticles (SKL-C1) achieved a 6.5-fold increase. This superior performance can be attributed to the higher syringyl (S) unit content and abundant methoxy groups in EKL-C1, which can improve UV absorption efficiency. Atomic force microscopy (AFM) further demonstrated smoother surface morphologies for EKL-C1-containing films, indicating improved nanoparticle dispersion and reduced aggregation. Mechanical testing before and after UV exposure confirmed the suitability of the films for packaging applications. These findings highlight the potential of lignin-based nanocomposite films as eco-friendly packaging and coating materials, offering a unique combination of high transparency and robust UV protection while, providing valuable insights into the structure–property relationships of lignin nanoparticles in biodegradable polymer films.