Faceted silver nanostructures including triangular nanoprisms, nanotetrahedra, and nanodecahedra were synthesized via a facile photochemical method at controlled wavelengths using spherical nanoparticles as the seeds. Scanning transmission electron microscopy studies showed that the resulting nanostructures were much larger in size (20-50 nm) than the spherical seed nanoparticles (under 5 nm), and X-ray diffraction as well as high-resolution transmission electron microscopy measurements confirmed that these nanostructures exhibited predominantly {111} faceted surfaces. Importantly, the silver nanostructures demonstrated markedly better antimicrobial activity than the spherical seed nanoparticles as evidenced by a lower minimum inhibitory concentration and more dramatic changes in both growth rate and lag phase at lower concentrations, which were attributed to the greater reactivity of the {111} faceted surfaces toward oxygen-rich bacterial surface moieties that allowed for more rapid localization to bacterial cells and increased interactions with structurally vital outer-membrane proteins. These results highlight the significance of surface morphologies of metal nanostructures in the manipulation of their antimicrobial activity.