Lignin is a common and abundant byproduct of the pulp and paper industry and is generally burned to produce steam. Opportunities exist to acquire greater value from lignin by leveraging the properties of this highly conjugated biomacromolecule for applications in UV absorption and polymer reinforcement. These applications can be commercialized by producing value-added lignin nanoparticles (LNPs) using a scalable sonochemical process. In the present research, monodisperse LNPs have been synthesized by subjecting aqueous dispersions of alkali lignin to acoustic irradiation. The resulting particle size distribution and colloidal stability, as determined by dynamic light scattering, transmission electron microscopy and zeta potential analysis, of LNPs can be adjusted by varying the solution pH and ultrasonication energy. As-synthesized LNPs with a mean diameter of 204 nm were incorporated into poly (vinyl) alcohol (PVA) to prepare thin and flexible nanocomposite films using a simple solvent casting method. The addition of 2.5 wt% LNP increased the material's Sun Protection Factor up to 26 compared to 0 for neat PVA, while maintaining light transmission above 75 % in the visible spectra. In addition, the tensile strength and elastic modulus of the PVA nanocomposites improved by 47 % and 36 %, respectively. The presence of LNP also enhanced the thermal stability of the materials. Significantly, the proposed sonochemical process may be generally applicable to the synthesis of a range of naturally-derived LNPs for a variety of value-added applications.