While Andreev bound states (ABSs) have been realized in engineered superconducting junctions, their direct observation in normal metal-superconductor heterostructures—enabled by quantum confinement—remains experimentally elusive. Here, we report the detection of ABSs in ultrathin metallic islands (Bi, Ag, and SnTe) grown on the $s$-wave superconductor NbN. Using high-resolution scanning tunneling microscopy and spectroscopy, we clearly reveal in-gap ABSs with energies symmetric about the Fermi level. While the energies of these states show no position dependence, their wave functions exhibit spatial oscillations, demonstrating a quantum size effect. Both the energy levels and spatial distribution of the ABSs can be reproduced by our effective model in which a metallic island is coupled to the superconducting substrate via the proximity effect. We demonstrate that the coupling strength plays a critical role in determining the ABS energies. Our work introduces a novel physical platform for implementing ABSs, which hold promise for significant device applications.

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• Andreev bound states
• Proximity effect
• Superconductivity
• Superconductors
• Bogoliubov-de Gennes equations
• Scanning tunneling microscopy
• Scanning tunneling spectroscopy