Aluminum nanocrystals and fabricated nanostructures are emerging as highly promising building blocks for plasmonics in the visible region of the spectrum. Even at the individual nanocrystal level, however, the localized plasmons supported by Al nanostructures possess a surprisingly broad spectral response. We have observed that when an Al nanocrystal is coupled to an underlying Al film, its dipolar plasmon resonance linewidth narrows remarkably and shows an enhanced scattering efficiency. This behavior is observable in other plasmonic metals, such as gold; however, it is far more dramatic in the aluminum nanoparticle−film system, reducing the dipolar plasmon linewidth by more than half. A substrate-mediated hybridization of the dipolar and quadrupolar plasmons of the nanoparticle reduces the radiative losses of the dipolar plasmon. While this is a general effect that applies to all metallic nanoparticle−film systems, this finding specifically provides a new mechanism for narrowing plasmon resonances in aluminum-based systems, quite possibly expanding the potential of Al-based plasmonics in real-world applications