1) Phosphorus-doped carbon nitride quantum dots (CNPQDs) were synthesized and decorated on square-shaped TiO2 nanotube arrays (STNAs) via a hydrothermal method to create a CNPQD-STNA composite. 2) Characterization of the CNPQD-STNA composite showed the CNPQDs were uniformly deposited on the STNA with an amorphous structure. 3) Photoelectrochemical testing demonstrated the CNPQD-STNA composite exhibited higher photocurrent density and incident photon-to-current efficiency for visible light-driven water splitting compared to pristine STNAs or TiO2 nanotube arrays coated with undoped C