Tetrahexahedral Pd-Ag alloy nanocrystals enclosed by high-index {520} facets with enhanced electrocatalytic activity have been synthesized using an Ag(i)-assisted growth protocol. The underpotential deposition of Ag plays dual roles in the formation of tetrahexahedral nanocrystals: it not only stabilizes the high-index {520} facets, but also facilitates the formation of Pd-Ag alloys.