A new class of gold nanostructures, concave nanocubes, enclosed by 24 high-index (720) facets, have been prepared in a monodisperse fashion by a modified seed-mediated synthetic method. The Cl- counterion in the surfactant plays an essential role in controlling the concave morphology of the final product. The concave nanocubes exhibit higher chemical activities compared with low-index {111}-faceted octahedra.