Graphitic　carbon　nitride　(g-C　3　N　4　)　is　a　kind　of　two-dimensional　material　with　unique　properties,　and　can　be　facilely　prepared　from　low-cost　precursors.　It　has　promising　application　in　CO　2　capture.　However,　the　low　alkalinity　of　nitrogen　species　in　g-C　3　N　4　makes　its　interaction　with　CO　2　very　weak.　Aiming　to　promote　the　use　of　g-C　3　N　4　for　CO　2　capture,　a　class　of　amine-functionalized　g-C　3　N　4　adsorbents　were　designed　in　the　present　work.　The　amine-functionalized　g-C　3　N　4　adsorbents　were　prepared　by　physical　impregnation　of　polyethylenimine　(PEI),　which　contains　many　amine　groups　and　exhibits　extremely　low　volatility,　into　the　stacked　pores　of　g-C　3　N　4　.　It　is　found　that,　after　the　amine　functionalization,　the　performance　of　g-C　3　N　4　for　CO　2　capture　(including　capacities　and　selectivities)　are　significantly　improved.　The　highest　CO　2　capacity　of　PEI-g-C　3　N　4　composites　is　3.77　mmol/g　when　determined　at　100　°C　and　ambient　pressure,　being　superior　to　most　of　those　of　other　g-C　3　N　4　materials　and　solid　amines　reported　in　the　literature.　Furthermore,　the　CO　2　adsorbed　by　PEI-g-C　3　N　4　composites　can　be　facilely　stripped　out　by　concentration　swing,　and　the　loss　in　CO　2　capacity　is　negligible　after　10　cycles　of　adsorption　and　desorption.　©　2018　American　Chemical　Society.