Roasted hazelnut skins (RHSs), generated as by-products of industrial hazelnut processing, were extracted by pressurized liquid extraction to yield a hydroalcoholic extract (RHS-H). The extract was rich in polyphenols (308.4 ± 4.6 mg GAE/g) and proanthocyanidins (169.3 ± 10 mg CE/g) and showed strong antioxidant activity (DPPH EC50 = 5.08 ± 1.08 µg/mL; TEAC = 2.82 ± 0.03 mM Trolox/mg) together with antimicrobial effects against Staphylococcus aureus and Staphylococcus epidermidis. RHS-H also enhanced the UV absorbance of synthetic UV filters. When incorporated into oil-in-water (O/W) cosmetic emulsions at low concentrations (0.2–2% w/w), RHS-H did not affect physicochemical stability: formulations maintained acceptable organoleptic properties, dermocompatible pH (4.7–5.5), electrostatic stability (ζ-potential ranging from –57 to –60 mV), and rheological behavior. Functionally, RHS-H increased the antioxidant activity of emulsions (radical scavenging > 80% vs. 52% in the blank), ensured microbial protection in challenge tests, and enhanced SPF by 9.4% at 0.2% w/w, with further improvements at higher concentrations, reaching broad-spectrum photoprotection (critical wavelength > 370 nm). Overall, RHS-H represents a natural multifunctional ingredient with antioxidant, preservative, and photoprotective properties, providing a sustainable strategy to upcycle hazelnut processing waste and reduce reliance on synthetic additives in cosmetic formulations.