We　synthesized　a　metal-free　photocatalyst　comprising　mesoporous　carbon　nitride　(mpg-C3N4)　and　carbon　nanotubes　(CNT),　via　a　facile　method　involving　polymerization　of　melamine　as　the　precursor　in　the　presence　of　CNT　in　refluxing　methanol.　Under　visible　light,　the　CNT/mpg-C3N4　composite　resulted　in　a　greater　than　8.6　×　faster　rhodamine　B　photodecomposition　rate　compared　with　bulk　graphitic　C3N4,　and　1.6　×　faster　compared　with　pure　mpg-C3N4.　We　attribute　the　promoted　photocatalytic　performance　of　the　composite　to　the　synergistic　effects　of　open-ended　CNT　and　mpg-C3N4.　The　highly　interconnected　CNT　in　mpg-C3N4　effectively　enhances　the　transfer　of　photo-generated　electrons,　harvests　more　incident　visible　light,　and　provides　specific　channels　for　oxygen　adsorption　and　photoreactions.　We　propose　a　possible　reaction　mechanism　and　report　a　scavenger　experiment　that　found　that　superoxide　radical　(·O2−)　is　mainly　responsible　for　dye　photodegradation.　The　CNT/mpg-C3N4　retained　good　stability　after　several　cycles.　This　work　supplies　a　promising　method　of　exploiting　a　stable,　metal-free,　efficient,　and　visible　light-driven　semiconductor　to　enhance　solar　energy　use　for　environmental　remediation.　©　2021,　The　Author(s),　under　exclusive　licence　to　Springer　Nature　B.V.