The detection of lactic acid with ZnO nanorod-gated AlGaN/GaN high electron mobility transistors (HEMTs) was demonstrated. The array of ZnO nanorods provided a large effective surface area with a high surface-to-volume ratio and a favorable environment for the immobilization of lactate oxidase. The HEMT drain-source current showed a rapid response when various concentrations of lactic acid solutions were introduced to the gate area of the HEMT sensor. The HEMT could detect lactic acid concentrations from 167 nM to 139 μM. Our results show that portable, fast response, and wireless-based lactic acid detectors can be realized with AlGaN/GaN HEMT based sensors.