Herein,　a　series　of　N/Ti3+-codoped　triphasic　TiO2/g-C3N4　heterojunctions　were　constructed　by　a　simple　hydrothermal　treatment　of　TiN　and　g-C3N4　and　systematically　investigated　by　a　number　of　characterization　techniques.　During　the　hydrothermal　reaction,　TiN　is　completely　converted　to　a　N/Ti3+-codoped　triphasic　heterophase　junction,　TiO2,　and　the　activated　g-C3N4　can　form　heterojunctions　with　TiO2.　Importantly,　a　red-shift　of　the　absorption　edge,　enhanced　absorption　intensity,　and　improved　photocatalytic　activity　were　observed　for　the　N/Ti3+-codoped　triphasic　TiO2/g-C3N4　heterojunctions　as　compared　to　the　case　of　N/Ti3+-codoped　triphasic　TiO2　and　pure　TiO2.　Especially,　the　N/Ti3+-codoped　triphasic　TiO2/g-C3N4　heterojunctions　with　an　optimal　molar　ratio　of　1 : 0.025　exhibited　highest　visible-light　photocatalytic　activity　for　the　degradation　of　methylene　blue,　rhodamine　B,　phenol,　and　levofloxacin.　Furthermore,　the　photoelectrochemical　and　electrochemical　impedance　spectra　measurements　revealed　that　the　prepared　heterojunctions　displayed　more　efficient　electron-hole　separation.　The　enhanced　photoactivity　is　attributed　to　the　synergistic　effect　of　integrated　factors,　including　N/Ti3+　doping　levels,　triphasic　heterophase　junctions,　and　TiO2/g-C3N4　heterojunctions,　which　account　for　the　improved　charge　separation　efficiency　and　extended　light　absorption　range　and　intensity.　In　this　study,　a　novel　structure　of　TiO2-based　heterojunction　photocatalysts　was　developed　by　a　simple　one-step　hydrothermal　approach,　and　an　efficient　route　to　improve　the　photocatalytic　activity　of　TiO2　was　revealed.　©　The　Royal　Society　of　Chemistry　and　the　Centre　National　de　la　Recherche　Scientifique.