The synthesis of core/shell perovskite nanocrystals was demonstrated for the first time by applying a seeded growth approach. The obtained core/shell nanocrystals were shown to consist of a light-emitting CsPbX3 core with a narrow band gap and a Cs4PbX6 passivating shell with a large band gap (>3.2 eV), and to therefore display an improved photoluminescence quantum yield.