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Fano resonance with an asymmetric and ultrasharp resonant line shape has been extensively studied in various light scattering scenes, unlocking several applications for sensing, information processing, and optical identification. Fano resonance appearing in multilayered nanoparticles (NPs) is particularly intriguing as its sharp and comb-like resonant line shape may enable optical identification at the nanoscale. We herein propose the concept of the optical physical unclonable function (PUF) based on the scattering responses of core–shell (plasmonic-dielectric) NPs. Specifically, the sharp, asymmetric spectral responses near the Fano resonance frequency, which are highly sensitive to perturbations (e.g., nanomanufacturing imperfections), can be exploited as a unique electromagnetic fingerprint for PUF-based identification and anti-counterfeiting applications. Here, we theoretically and statistically demonstrate that scattering from Fano-resonant multilayered NPs can be regarded as a perfect entropy source for the generation of PUF encryption keys, with outstanding performance in terms of uniqueness, randomness, encoding capacity, and NIST randomness test results. The proposed optical PUF opens pathways to implement nano-tags for optical identification, authentication, and anti-counterfeiting applications.