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Quantum confinement is known to affect a nanosized superconductor through quantum‐size variations of the electronic density of states. Here, it is demonstrate that there is another quantum‐confinement mechanism overlooked in previous studies. In particular, it is found that the electron–electron attraction can be enhanced due to quantum‐confinement modifications of electronic wave functions. The superconducting correlations are strengthened by such quantum mechanical effect, which creates a subtle interplay with surface–substrate phonon modifications. The combined effect depends on nanofilm thickness and can be controlled by nanoarchitechture. The calculations are in a reasonable agreement with experiments performed on high‐quality aluminum films. These findings shed light on the long‐standing problem of the size dependence of the critical temperature in low‐dimensional superconductors. Nanostructured superconductors attract significant interest because their properties may strongly vary with the system geometry and dimensions. This opens prospects of architecting superconducting nanodevices in order to tune their characteristics. Here the authors demonstrate that one of promising examples is high‐quality metallic nanofilms, where superconductivity is enhanced by quantum‐size effects and the enhancement is well controlled by the nanofilm thickness. [ABSTRACT FROM AUTHOR]