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Background: Incidental thyroid nodules (ITNs) are defined as newly encountered nodules identified on imaging performed for an unrelated purpose. In practice, ITNs are often detected on chest computed tomography (CT). We investigated the prevalence and clinical significance of ITNs detected on low-dose chest CT (LDCT) for lung cancer screening. Methods: We retrospectively reviewed the electronic medical records of patients with no history of thyroid disease who underwent LDCT for lung cancer screening between March 2009 and February 2012 at Jeju National University Hospital (Korea). Results: Among 1,941 patients that underwent LDCT, 55(2.8%) were found to have ITNs. Seven (12.7%) of those cases were malignant. The positive and negative predictive values of chest LDCT for the detection of incidental malignant thyroid nodules were 26.9% and 73.4%, respectively. Factors considered to be predictive of malignancy on LDCT were a mean attenuation value of 55 HU or more (p = 0.036) and the presence of dense calcifications (p = 0.048). Sex, age, location of the nodule, longest diameter of the lesion, AP/T (anteroposterior/transverse dimension) ratio, margins, density, presence of punctate calcifications, and thyroid enlargement had no significant predictive value in discriminating benign and malignant nodules. On multivariate analyses, a mean attenuation value above 55 was the only statistically significant feature (p = 0.048). Conclusions: A mean attenuation value greater than 55 HU on LDCT may be a useful predictive factor for differentiating malignant from benign lesions. Therefore, a careful assessment of the thyroid gland is necessary for patients undergoing LDCT for lung cancer screening.