Background and objective Many bronchiectasis patients suffer dyspnea, decreased exercise tolerance, and low body mass index. Chest wall muscles play a special role in respiratory movement and make up parts of skeletal muscles. This study aimed to examine the chest wall muscle thickness and their relationship with disease severity in bronchiectasis. Methods We retrospectively included 166 patients with bronchiectasis and 62 patients with pneumonia as comparators. The thickness of chest wall muscle as determined in chest CT, pulmonary function, and Bronchiectasis Severity Index (BSI) score were recorded. We compared the thickness of the chest wall muscle in two groups and assessed the relationships among chest wall muscle thickness, pulmonary function, and BSI score. Results Chest wall muscle thickness of the anterior midclavicular line and posterior exterior scapula were thinner in bronchiectasis patients than comparators both above the aortic arch level and at the aortic arch window level. Muscle thickness of the posterior interior scapula above the aortic arch level was significantly thinner in bronchiectasis patients. Chest wall muscle thickness at the anterior midclavicular line both the above aortic arch level and at the level of the aortic arch window were related to diffuse capacity in bronchiectasis patients. Anterior chest wall muscle thickness above the aortic arch was found to be a risk factor of disease severity. Conclusion Anterior chest wall muscles in the upper and middle chest were thinner in bronchiectasis patients than in comparators, and had relationship with spirometry and diffuse compacity factors. We provide another method to conveniently assess bronchiectasis severity.