A size- and shape-selective synthesis of pentagonally twinned silver icosahedral nanoparticles (AgI_hNPs), one of the five platonic solid morphologies, has been developed by integrating three key factors: nuclei templating by copper, photochemical development using violet LED light and chemical oxidative etching. The presence of copper is essential for AgI_hNP shape selection via the promotion of icosahedral nuclei in precursor NPs. Violet light (401–410 nm) is crucial to promote plasmonic selection of near-spherical AgI_hNPs. Oxidative etching with hydrogen peroxide and photochemical reduction with citrate establishes a red-ox equilibrium for the photochemical selection of AgI_hNPs. The addition of chloride ions improves size- and shape-selectivity. Finally, the demonstration of 1-D growth of AgI_hNPs to pentagonal pins initiated at AgI_hNP pentagonal-twinned defects highlights a universal role of twinned defects for the formation of anisotropic nanoparticles.