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Introduction/Background Measurement of cervical spine range of motion (ROM) is often considered to be an essential component of cervical spine physiotherapy assessment. This study aimed to investigate the reliability and validity of an iPhone application (app) (Goniometer Pro) for measuring active craniocervical ROM (ACCROM) in patients with nonspecific neck pain. Material and method A cross-sectional study was conducted at the musculoskeletal biomechanics laboratory located at Iran University of Medical Sciences. Forty nonspecific neck pain patients participated in this study. The outcome measure was the ACCROM, including flexion, extension, lateral flexion, and rotation. Following the recruitment process, ACCROM was measured using a universal goniometer (UG) and iPhone 7 app. Two blinded examiners each used the UG and iPhone to measure ACCROM in the following sequences: flexion, extension, lateral flexion, and rotation. The second (2 hours later) and third (48 hours later) sessions were carried out in the same manner as the first session. Intraclass correlation coefficient (ICC) models were used to determine the intra-rater and inter-rater reliability. The Pearson correlation coefficients were used to establish concurrent validity of the iPhone app. Minimum detectable change at the 95% confidence level (MDC95) was also computed. Results Good intra-rater and inter-rater reliability was demonstrated for the goniometer with ICC values of ≥ 0.66 and ≥ 0.70 and the iPhone app with ICC values of ≥ 0.62 and ≥ 0.65, respectively. The MDC95 ranged from 2.21° to 12.50° for the intra-rater analysis and from 3.40° to 12.61° for the inter-rater analysis. The concurrent validity between the two instruments was high, with r values of ≥ 0.63. The magnitude of the differences between the UG and iPhone app values (effect sizes) was small, with Cohen d values of ≤ 0.17. Conclusion The iPhone app possesses good reliability and high validity. It seems that this app can be used for measuring ACCROM.