Background/Objectives: There is limited evidence regarding the impact of the coronavirus disease 2019 (COVID-19) pandemic on the epidemiology of thoracic and cardiovascular diseases. This study aimed to investigate changes in medical visits for these conditions during the COVID-19 pandemic. Methods: We analyzed the entire Korean population (~50 million) for monthly medical visits for 15 common thoracic and cardiovascular conditions, including pneumothorax, large bullae, lung cancer, esophageal cancer, thymoma, empyema, mediastinitis, esophageal rupture, multiple rib fractures, hemothorax, rib mass, varicose vein, pectus excavatum, aortic dissection, aortic aneurysm, and valve disease from January 2019 to December 2021. Data were obtained from the Korean National Health Insurance Service using the International Classification of Disease (ICD)-10 codes. Variations in the mean monthly medical visits of 15 frequent thoracic and cardiovascular diseases before and during the COVID-19 pandemic were compared using the Mann–Whitney U test, while changes in variance were assessed using Levene’s test. Results: The mean monthly number of medical visits for pneumothorax and large bullae significantly decreased during the COVID-19 pandemic compared to before the pandemic (by 10.1% and 12.8%; both p < 0.001). On the contrary, there was a significant increase in the mean monthly counts of medical visits for lung cancer, esophageal cancer, thymoma, and valve disease diagnosis (by 6.6%, 5.3%, 8.8%, and 5.0%, respectively; all p < 0.05). Conclusions: In Korea, the number of diagnosed cases of pneumothorax significantly decreased during the COVID-19 pandemic compared to before COVID-19, while diagnoses of thoracic cancers and valve disease increased.