This study investigates lignin nanoparticles (LNPs) from spruce and Eucalyptus kraft lignins as sustainable additives for sunscreen formulations. The lignins and their nanoparticles were characterized using spectroscopic, chromatographic, and microscopic techniques and incorporated into oil-in-water sunscreen emulsions, where they were evaluated for UV-blocking efficiency, stability, and rheological properties. Results demonstrated that LNPs significantly enhanced UV protection, with spruce kraft lignin nanoparticles providing superior broad-spectrum coverage (280–400 nm) and an SPF of 12.94, compared to Eucalyptus lignin nanoparticles, which primarily absorbed in the UVB range (280–320 nm) and reached an SPF of 7.00. Additionally, LNPs improved emulsion stability through Pickering stabilization and enhanced rheological properties, making them promising eco-friendly and multifunctional sunscreen additives.