The formation of complexes between copper ions and biomolecules plays important roles in biological systems. In this work, the structures and electrochemical properties of copper-creatinine complexes were investigated by both experimental and computational approaches. DFT calculation revealed the possible structures of copper-creatinine complexes and provided the data of formation energies, bond lengths, and charge distribution. The properties of the complexes were further investigated by cyclic voltammetry, UV-visible spectrophotometry, X-ray absorption spectroscopy, and scanning electron microscopy. The combination of experimental and computational findings revealed that CuII binds with creatinine via the endocyclic nitrogen. In aqueous environment, the [Cu(creatinine)2(H2O)2]2+ complex is formed. The reduction of [Cu(creatinine)2(H2O)2]2+ formed a stable 1:4 complex between CuI and creatinine. Importantly, the understanding of the electrochemical behaviors of copper-creatinine complexes leads to the development of a novel sensor for the detection of creatinine, a biomarker for kidney diseases. Although creatinine itself is not electroactive, the complex formation with copper allows the species to be detected electrochemically with the sensitivity of 6.09 ± 0.13 µA mM−1 and the limit of detection (3sB/m) of 35 µM.