Yolk–shell　nanoreactors　have　received　considerable　interest　for　use　in　catalysis.　However,　the　controlled　synthesis　of　continuous　crystalline　shells　without　imperfections　or　cracks　remains　challenging.　Here,　a　strategy　for　the　synthesis　of　yolk–shell　metal　nanoparticles@covalent　organic　framework　(MNPs@COF)　nanoreactors　by　using　MNPs@ZIF-8　core–shell　nanostructures　as　a　self-template　is　designed　and　developed.　The　COF　shell　is　formed　through　an　amorphous-to-crystalline　transformation　process　of　a　polyimine　shell　in　a　mildly　acidic　solution,　while　the　ZIF-8　is　etched　in　situ,　generating　a　void　space　between　the　MNPs　core　and　the　COF　shell.　With　the　protection　of　the　COF　shell,　multiple　ligand-free　MNPs　are　confined　inside　of　the　hollow　nanocages.　Importantly,　the　synthetic　strategy　can　be　generalized　to　engineer　the　functions　and　properties　of　the　designed　yolk–shell　nanocages　by　varying　the　structure　of　the　COF　shell　and/or　the　composition　of　the　core　MNPs.　Representative　Pd@H-TpPa　yolk–shell　nanocages　with　active　Pd　NP　cores　and　permeable　TpPa　shells　exhibit　high　catalytic　activity　and　stability　in　the　reduction　of　4-nitrophenol　by　NaBH4　at　room　temperature.　©　2018　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim