Reinforcement steel corrosion is one of the primary causes of premature deterioration of reinforced concrete (RC) structures in chloride-rich environments. Early detection of steel corrosion enables timely maintenance measures, not only to reduce the subsequent costs associated with corrosion propagation but also to ensure the safe operation of RC structures. Due to its high sensitivity, compactness, cost-effectiveness, and capability of being connected in a network, various fiber optic corrosion sensors have been developed for steel reinforcement corrosion monitoring in RC structures. Unlike grating-based optical fiber sensors that require an expensive manufacturing process, the no-core fiber (NCF) optic sensor has a simple structure and has been proposed for corrosion monitoring of steel rebar recently. In this study, the NCF fiber sensor was characterized for early-stage corrosion monitoring of steel rebar. A segment of NCF fiber with a length of approximately 20 mm was fusion spliced between two single-mode fibers and then attached to the steel bar surface. The steel rebar was embedded in a mortar block. After curing for 28 days, the mortar specimen was submerged in a 3.5 wt.% NaCl solution for corrosion tests. During the immersion, the shift in the transmission spectrum of the NCF sensor was recorded using an optical spectrum analyzer, while the corrosion evolution of the steel rebar was measured with linear polarization resistance. A relationship between the shift in the transmission spectrum of the NCF sensor and the corrosion-induced mass loss of the steel rebar was established, which can be used for corrosion monitoring of steel rebar.