A novel multifunctional sensor is developed for in situ and non-destructive monitoring of the corrosion current and open circuit potential of reinforcing steel, as well as the pH and Cl? concentration of concrete. The pH and Cl? sensors show good responses to the pH and Cl? concentration of concrete pore solutions, respectively, and are able to monitor both the carbonization process of concrete and the ingress of Cl? in concrete. Combined with measurements of the corrosion potential and corrosion current density, as well as the EIS spectra of reinforcing steel in concrete, this study demonstrates that the pH and the Cl? concentration of concrete are two of the most crucial factors that determine the corrosion of reinforcing steel in concrete.