We analyzed the inter- and intra-examiner reliability of Werium inertial sensors and the cervical range of motion (CROM) instrument for the measurement of active CROM (AcROM) in patients with primary headache. Another objective is to analyze the validity of the inertial sensors (Werium). The literature has reported symptomatology features in patients diagnosed with primary headache similar to that of patients with cervicogenic headache. The International Classification of Headache (ICHD-III) established the presence of reduced AcROM as a diagnostic criterion for cervicogenic headache. Several instruments are used for this measurement, with limitations in their applicability in daily clinical practice. A prospective longitudinal repeated measures study was conducted to assess the intra- and inter-rater reliability and validity of Werium inertial sensors in 20 adults with chronic primary headache. For the inter-rater analysis, the intraclass correlation coefficient (ICC) values were above 0.75 for all movements, indicating a good level of reliability. For the intra-rater results, the ICC values obtained by the Werium inertial sensors for all cervical movements were good for rater A (ICC >0.80) and rater B (ICC >0.84). For the validity, the ICCs obtained by the Werium inertial sensors compared with the CROM instrument for all cervical movements were moderate for both raters (ICC > 0.70, respectively). Values obtained in the standard error of measurement, minimum detectable change at 90% and limits of agreement also indicated good agreement. Werium inertial sensors have shown good to excellent reliability results, both intra- and inter-examiner (ICC > 0.75). Likewise, when the sensors were compared with another validated instrument (CROM device) they obtained high reliability results (ICC > 0.70). These results plus its relatively low price and ease of use allow us to recommend it in daily clinical practice to measure AcROM in patients with chronic primary headache.