Cervical active range of motion (AROM) is an important outcome measure for clinicians working with a range of patient populations, especially people with neck pain. Multi-sensor inertial measurement unit (IMU) devices demonstrate good validity in the research laboratory but are expensive and not easily accessible in clinical settings. The use of single-IMU devices has been proposed but their validity for measuring cervical AROM is unknown. A concurrent and content validity study was conducted, comparing accuracy of single-IMU NeckCare Pro™ with multi-IMU Xsens™ for measuring cervical AROM in healthy adults (8 males, 7 females, mean age 30.6 years [SD 10.4]). Cervical AROM was assessed for flexion, extension, rotation (right and left), and lateral flexion (right and left), whereby six repetitions were performed for each movement with the subjects strapped to a high-back chair. Regarding content validity, Xsens™ detected a small amount of thoracic movement that could not be detected by the NeckCare Pro™ during cervical AROM testing, with means ranging from 1.5° to 4.1°. However, this did not significantly impact concurrent validity, which was good for all movements (ICC 0.764 – 0.966). This paper found that single-IMU technology (NeckCare Pro™) had good validity for measuring cervical AROM in healthy adults when subjects were strapped to a chair to limit trunk movement.