Increasing　the　active　sites　upon　single-atom　catalysts　is　an　optional　strategy　to　increase　the　catalyst　efficiency.　In　this　study,　the　loading　of　three　and　four　Group　VIII　and　Group　IB　transition　metal　(TM)　atoms　on　poly(triazine　imide)　(PTI)　nanosheets　was　firstly　explored.　Meanwhile,　comprehensive　first-principles　investigations　were　carried　out　to　explore　their　structural　stability,　electronic　properties,　and　catalytic　performances　in　the　context　of　the　hydrogen　evolution　reaction　(HER).　Our　results　indicate　that　11　catalysts　are　suitable　candidates　for　HER　applicability　among　the　total　38　catalysts.　Notably,　the　4TM@H-PTI　catalysts　have　more　active　sites　than　3TM@H-PTI　and　show　more　effective　electron　transfer　with　the　substrate.　Consequently,　the　4TM@H-PTI　catalysts　can　be　regarded　as　novel　and　highly　prospective　candidates　for　HER　applications.　The　present　work　provides　valuable　insights　and　paves　the　way　for　the　design　of　co-catalysts　on　PTI　that　are　both　low-cost　and　high-performance,　potentially　creating　novel　opportunities　for　progress　within　the　realm　of　photocatalysis　and　sustainable　energy.　©　2025　Elsevier　B.V.