In this paper, a microwave sensor based on a circular substrate integrated waveguide (CSIW) structure is introduced for detecting blood glucose levels by determining the resonant frequency of the scattering parameters. The scattering parameters were examined in the presence of glucose samples with various concentrations over a frequency range of 5–10 GHz (centered at 9 GHz). The sensor’s operation relies on changes in the permittivity of blood associated with glucose concentration, which is modeled using a Debye dispersion model. The principal design challenge is to tune the resonant sensor so that it is highly sensitive, even to very small sample volumes. This sensitivity must manifest as a discernible and distinguishable shift in the resonance frequency for different blood glucose concentrations. The sample volume in the proposed sensor is only 30 µL, which lies in the microfluidic regime and is smaller than that of a standard drop of blood. Simulation results for common blood glucose concentrations (50–500 mg/dL) show a significant resonant frequency shift (~100 MHz), indicating strong differentiation between glucose levels.The sensor achieves an accuracy of 99.12% with an average measurement error below ±0.4%, outperforming similar sensors. An unloaded quality factor greater than 450 is obtained, ensuring precise measurement capability. Prominent features of this sensor include high sensitivity, a relatively simple design and fabrication process, fast response time, and cost-effectiveness. Overall, the proposed sensor demonstrates high sensitivity and stable response in blood glucose detection, with its high accuracy and rapid response making it well-suited for noninvasive monitoring applications.