Authentication of persons and objects is a crucial aspect of security. We experimentally demonstrate quantum-secure authentication (QSA) of a classical multiple-scattering key. The key is authenticated by illuminating it with a light pulse containing fewer photons than spatial degrees of freedom and verifying the spatial shape of the reflected light. Quantum-physical principles forbid an attacker to fully characterize the incident light pulse. Therefore, he cannot emulate the key by digitally constructing the expected optical response, even if all information about the key is publicly known. QSA uses a key that cannot be copied due to technological limitations and is quantum-secure against digital emulation. Moreover, QSA does not depend on secrecy of stored data, does not depend on unproven mathematical assumptions, and is straightforward to implement with current technology.