Unique single-crystalline octahedral Cu2O nanocages (see Figure) are synthesized in solution by the catalytic reduction of copper tartrate complex into octahedral Cu2O nanocrystals and a subsequent spontaneous hollowing process. A wealth of colorful nanostructures with widely tunable bandgaps in the range 2.6-2.2 eV are obtained. The obtained nanocages may find potential use in solar-energy conversion, catalysis, and as model systems for fundamental research.