Photoinduced　reactive　oxygen　species　(ROSs)-involving　nitrogen　oxide　(NOx)　abatement　offers　a　green　way　to　mitigate　environmental　air　pollution.　Developing　cost-effective　photocatalysts　remains　challenging　yet　crucial.　Herein,　an　atomically　dispersed　Mg-modified　g-C3N4　nanosheet　photocatalyst　(xMg-CN)　with　excellent　photoactivity　is　developed.　The　1.0Mg-CN　sample　delivers　a　high　NO　removal　rate　of　52.3%　under　visible　light　irradiation　within　30　min,　far　surpassing　the　Mg-free　analog　(CN,　41.2%).　Moreover,　a　suppressed　NO2　by-product　discharge　can　be　achieved.　The　introduction　of　Mg　single　atoms　(SAs)　alters　the　charge　density　distribution　on　the　triazine　ring　plane　of　g-C3N4,　inducing　the　formation　of　a　built-in　electric　field,　which　improves　the　photoexcited　charge　carrier　separation.　Additionally,　Mg　SAs　contribute　to　the　adsorption　and　activation　of　O2　molecules,　resulting　in　enhanced　ROSs　production,　as　evidenced　by　the　experiments　and　theoretical　simulations.　The　findings　shed　light　on　the　role　of　SAs　in　designing　highly　efficient　g-C3N4-based　photocatalysts　for　air　purification.　©　2024　Elsevier　B.V.