Carbon-supported　metal　nanoparticles　(NPs)　comprise　an　important　class　of　heterogeneous　catalysts.　The　interaction　between　the　metal　and　carbon　support　influences　the　overall　material　properties,　viz.,　the　catalytic　performance.　Herein　we　use　in　situ　and　ex　situ　transmission　electron　microscopy　(TEM)　in　combination　with　in　situ　X-ray　spectroscopy　(XPS)　to　investigate　the　encapsulation　of　metallic　iridium　NPs　by　carbon　in　an　Ir/C　catalyst.　Real-time　atomic-scale　imaging　visualizes　particle　reshaping　and　increased　graphitization　of　the　carbon　support　upon　heating　of　Ir/C　in　vacuum.　According　to　in　situ　TEM　results,　carbon　overcoating　grows　over　Ir　NPs　during　the　heating　process,　starting　from　ca.　550　°C.　With　the　carbon　overlayers　formed,　no　sintering　and　migration　of　Ir　NPs　is　observed　at　800　°C,　yet　the　initial　Ir　NPs　sinter　at　or　below　550　°C,　i.e.,　at　a　temperature　associated　with　an　incomplete　particle　encapsulation.　The　carbon　overlayer　corrugates　when　the　temperature　is　decreased　from　800　to　200　°C　and　this　process　is　associated　with　the　particle　surface　reconstruction　and　is　reversible,　such　that　the　corrugated　carbon　overlayer　can　be　smoothed　out　by　increasing　the　temperature　back　to　800　°C.　The　catalytic　performance　(activity　and　stability)　of　the　encapsulated　Ir　NPs　in　the　hydrogen　evolution　reaction　(HER)　is　higher　than　that　of　the　initial　(nonencapsulated)　state　of　Ir/C.　Overall,　this　work　highlights　microscopic　details　of　the　currently　understudied　phenomenon　of　the　carbon　encapsulation　of　supported　noble　metal　NPs　and　demonstrates　additionally　that　the　encapsulation　by　carbon　is　an　effective　measure　for　tuning　the　catalytic　performance.　©　2023　American　Chemical　Society.