Abstract: Flexible respiratory monitoring technology has shown considerable clinical potential for the early diagnosis of diseases such as chronic obstructive pulmonary disease and sleep apnea. In this study, a highly sensitive and ultrafast response flexible breath sensor based on B-phase vanadium dioxide nanobelts (VO₂(B) NBs) is proposed. The VO₂(B) NBs, synthesized via the hydrothermal method, are active semiconductor materials with a large specific surface area. These nanobelts exhibit high sensitivity to humidified air and can detect relative humidity (RH) ranging from 26% to 93% at room temperature. By integrating VO₂(B) NBs with a top-layer Au interdigitated electrode and a bottom-layer flexible PI substrate, a flexible breath sensor was developed. This sensor demonstrated significant resistance variation, with a response time of 0.32 s and a recovery time of 0.41 s during exhalation and inhalation. Cyclic experimental results confirm that the sensor exhibits excellent consistency and repeatability in tracking human breath across various breathing rates, suggesting its suitability for long-term monitoring. The superior sensing performance and rapid response time indicate that the VO₂(B) NBs-based breath sensor is a viable solution for monitoring respiration to prevent sleep apnea syndrome.