Copper ions (Cu²⁺) play an essential role in various cellular functions, including respiration, nerve conduction, tissue maturation, oxidative stress defense, and iron metabolism. Covalent organic frameworks (COFs) are a class of porous crystalline materials with directed structural designability and high stability due to the combination of different monomers through covalent bonds. In this study, we synthesized a porphyrin-tetrathiazole COF (TT-COF(Zn)) with Zn-porphyrin and tetrathiafulvalene (TTF) as monomers and used it as a photoactive material. The strong light absorption of metalloporphyrin and the electron-rich properties of supplied TTF contribute to its photoelectrochemical performance. Additionally, the sulfur (S) in the TTF can coordinate with Cu²⁺. Based on these properties, we constructed a highly sensitive photoelectrochemical sensor for detecting Cu²⁺. The sensor exhibited a linear range from 0.5 nM to 500 nM (R² = 0.9983) and a detection limit of 0.15 nM for Cu²⁺. Notably, the sensor performed well when detecting Cu²⁺ in water samples.