The aim of this study was to evaluate the test–retest reliability of an integrated inertial sensor (IIS) for cervical range of motion assessment. An integrated inertial sensor was placed on the forehead center of thirty older adults (OA) and thirty younger adults (YA). Participants had to perform three continuous rotations, lateral bandings and flexion–extensions with their head. Test–retest reliability was assessed after 7 days. YA showed moderate to good agreement for rotation (0.54–0.82), lateral bending (0.74–0.8), and flexion–extension (0.74–0.81) movements and poor agreement for zero point (ZP). OA showed moderate to good agreement for rotation (0.65–0.86), good to excellent agreement in lateral bending (0.79–0.92), and poor to moderate agreement for flexion–extension (0.37–0.72). Zero point showed poor to moderate agreement. In conclusion, we can affirm that this IIS is a reliable device for cervical range of motion assessment in young and older adults; on the contrary, the ZP seems to be unreliable and the addition of an external reference point could help the subject to solve this shortcoming and reduce possible biases.