Hazelnut skin is a thin brown coating surrounding the kernel that separates during roasting and is usually discarded as a by-product. However, it is rich in dietary fiber and bioactive compounds that vary by cultivar and region. In this study, the chemical composition and aflatoxin levels of hazelnut skins from Kocaeli and Ordu provinces in Türkiye were analyzed, with phenolic contents evaluated only for the Ordu sample. In the Kocaeli sample, dry matter, ash, fat, and protein contents were 91.07%, 2.14%, 12.94%, and 8.72%, respectively; whereas for the Ordu sample they were 95.38%, 1.90%, 26.55%, and 15.03%. Fourier transform infrared spectroscopy (FTIR) indicated phenolic and lipid functional groups. Ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS) showed that isoquercitrin (436.69 µg/g), catechin (270.77 µg/g), and quinic acid (231.64 µg/g) were the major compounds, while rutin (49.09 µg/g), hesperidin (20.46 µg/g), and quercetin (13.87 µg/g) were detected at moderate levels, including some phenolics rarely reported in hazelnut skins. Total aflatoxin in the Ordu sample was below the EU maximum level (10 μg/kg), whereas the Kocaeli sample was close to this limit (9.99 μg/kg). These findings suggest that the Ordu sample contained higher fat, protein, and selected phenolics, supporting its potential use as a functional ingredient. Improved drying and monitoring practices may help reduce aflatoxin G₂ contamination. Further studies are required to confirm bioavailability and safety under different conditions.