Transition　metal　nanoparticles　encapsulated　in　carbon　nanostructures　(M@Carbon,　M　=　Fe,　Co,　Ni,　et　al.)　exhibit　excellent　catalytic　performance　toward　the　hydrogen　evolution　reaction　(HER).　However,　their　application　as　HER　cocatalysts　for　photocatalytic　hydrogen　production　is　greatly　hampered　by　their　random　growth　nanostructures　with　rather　big　overall　size　(e.g.,　nanotube　structure　with　length　of　5-50　μm).　Herein,　a　nanoconfined　assembly　strategy　is　well　developed　for　the　textural　engineering　of　Co@Carbon　with　an　optimized　nanostructure　for　photocatalysis.　The　spatial　compartmentalization　and　physical　confinement　of　precursors　within　the　silica　aerogel　kinetically　control　the　core-shell　nanostructure　assembly　and　facilitates　the　synthesis　of　Co@Carbon　with　controllable　morphology　and　reduced　particle　size　(∼30　nm),　making　Co@Carbon　a　promising　HER　cocatalyst　for　photocatalytic　hydrogen　evolution.　Furthermore,　this　nanoconfined　assembly　strategy　is　a　general　approach　that　can　be　widely　applied　for　textural　engineering　of　other　transition　metal-embedded　or　their　alloy-embedded　carbon　structures　for　photocatalysis　or　other　energy-related　applications.　Copyright　©　2019　American　Chemical　Society.