Neck pain is a common musculoskeletal disorder. Therefore, establishing effective physical therapy for neck pain is one of the most important issues. In addition, in physical therapy for neck pain, it is important to evaluate the thoracic spine, which is an adjacent region of the neck. The lumbar-locked rotation test is designed to evaluate the rotational range of the thoracic spine. However, the reliability of the test when performed on patients with neck pain has not been confirmed. We aimed to determine the intra- and inter-rater reliability of the lumbar-locked rotation test in patients with neck pain. In this study involving 43 patients, two separate examiners measured thoracic spine rotation. Both examiners conducted three measurements for each side, before and after a five-minute interval. Reliability was assessed using various intra-class correlation coefficient (ICC) models. The intra-rater reliability showed ICC values of 0.99 for both examiners. The inter-rater reliability showed ICC values of 0.98 for both right and left thoracic rotations. The findings strongly suggest that the lumbar-locked rotation test has high within-session intra- and inter-rater reliability for patients with neck pain. This test can be considered a reliable method of measuring the thoracic spine rotational range of motion in patients with neck pain in clinical practice.