Carbon　dots　on　photoactive　semiconductor　nanomaterials　have　represented　an　effective　strategy　for　enhancing　their　photoelectrochemical　(PEC)　activity.　By　carefully　designing　and　manipulating　a　carbon　dot/support　composite,　a　high　photocurrent　could　be　obtained.　Currently,　there　is　not　much　fundamental　understanding　of　how　the　interaction　between　such　materials　can　facilitate　the　reaction　process.　This　hinders　the　wide　applicability　of　PEC　devices.　To　address　this　need　of　improving　the　fundamental　understanding　of　the　carbon　dots/semiconductor　nanocomposite,　we　have　taken　the　TiO2　case　as　a　model　semiconductor　system　with　nitrogen-doped　carbon　dots　(NCDs).　We　present　here　with　in-depth　investigation　of　the　structural　hybridization　and　energy　transitions　in　the　NCDs/TiO2　photoelectrode　via　high-resolution　scanning　transmission　microscopy　(HR-STEM),　electron　energy　loss　spectroscopy　(EELS),　UV-vis　absorption,　electrochemical　impedance　spectroscopy　(EIS),　Mott-Schottky　(M-S),　time-correlated　single-photon　counting　(TCSPC),　and　ultraviolet　photoelectron　spectroscopy　(UPS),　which　shed　some　light　on　the　charge-transfer　process　at　the　carbon　dots　and　TiO2　interface.　We　show　that　N　doping　in　carbon　dots　can　effectively　prolong　the　carrier　lifetime,　and　the　hybridization　of　NCDs　and　TiO2　is　able　not　only　to　extend　TiO2　light　response　into　the　visible　range　but　also　to　form　a　heterojunction　at　the　NCDs/TiO2　interface　with　a　properly　aligned　band　structure　that　allows　a　spatial　separation　of　the　charges.　This　work　is　arguably　the　first　to　report　the　direct　probing　of　the　band　positions　of　the　carbon　dot-TiO2　nanoparticle　composite　in　a　PEC　system　for　understanding　the　energy-transfer　mechanism,　demonstrating　the　favorable　role　of　NCDs　in　the　photocurrent　response　of　TiO2　for　the　water　oxidation　process.　This　study　reveals　the　importance　of　combining　structural,　photophysical,　and　electrochemical　experiments　to　develop　a　comprehensive　understanding　of　the　nanoscale　charge-transfer　processes　between　the　carbon　dots　and　their　catalyst　supports.　Copyright　©　2020　American　Chemical　Society.