AbstractTuning the bandgap of a semiconductor to achieve strong band‐to‐band visible light absorption is highly desirable but challenging for photocatalysis. This work presents a facile molten‐salt‐assisted route to prepare red‐colored polymerized carbon nitride (RPCN) nanosheets with a remarkable redshifted absorption and narrowed bandgap of 1.9 eV. Both experimental findings and theoretical calculations reveal that alkali heteroatoms are effective to tune the surface and electronic structures of carbon nitride, resulting in significantly reduced bandgap and excellent solubility. The RPCN‐sensitized TiO2 nanorod‐based photoanode generates an impressive photocurrent density of ≈2.33 mA cm−1 at 1.23 V versus reversible hydrogen electrode under Air Mass 1.5 G illumination without any cocatalyst, which is 2.6 folds higher than that of the bare TiO2 photoanode. The new findings in this work could inspire the electronic structure engineering of semiconductor photocatalysts to greatly enhance the visible light absorption and provide a generic strategy to enhance the photoelectrochemical performance of wide‐bandgap semiconductor photoelectrodes.