Forward head posture (FHP) is one of the most common postural disorders affecting the head and neck region, typically assessed by measuring the craniovertebral angle (CVA). Selecting an appropriate tool for quantifying this angle is crucial. The primary aim of the present study was to evaluate the reliability of ImageJ software in assessing the CVA in participants with and without neck pain.The study included twenty participants with neck pain and twenty without. Reflective markers were placed on the tragus of the ear and the spinous process of C7. Three lateral images were captured from the dominant side while the participants were seated, with measurements taken at two-hour and one-week intervals. The craniovertebral angle (CVA) was then analyzed using ImageJ software.The Intraclass Correlation Coefficient (ICC) values for both within-day and between-day reliability in both groups ranged from 0.89 to 0.94. The Standard Error of Measurement (SEM) ranged from 0.92° to 1.02°, while the Minimal Detectable Change (MDC) values ranged from 2.56° to 2.84°. Furthermore, the absolute Technical Error of Measurement (TEM) ranged from 0.91° to 1.06°, and the relative TEM (rTEM) ranged from 2.13% to 2.42%.ImageJ software appears to be a suitable tool for assessing the craniovertebral angle (CVA) in individuals with or without neck pain, demonstrating good to excellent reliability.