Background: Coronary artery disease is a chronic condition that requires continuous adherence to healthy behaviors. Adhering to physical activity and reducing prolonged sedentary behavior are important for the physical health of older patients with coronary artery disease (CAD). Health literacy is increasingly recognized as a key factor in secondary prevention for these patients. However, evidence regarding the associations among health literacy, physical activity, and sedentary behaviors for secondary prevention in older patients with coronary artery disease remains limited. Objectives: This study aimed to identify the prevalence of limited health literacy and the influence of health literacy on physical activity and sedentary behaviors in older patients with CAD. Methods: This study was a descriptive cross-sectional study conducted from May to December 2022 in South Korea. A total of 186 patients aged 65 and older diagnosed with coronary artery disease participated. Data on health literacy, physical activity, and sedentary behaviors were collected through self-reported questionnaires and electronic medical records. Results: The prevalence of limited health literacy in older patients was 66.7%. Hierarchical linear regression revealed that health literacy was a significant determinant of physical activity (β = −0.50, P < .001) and sedentary behaviors (β = 0.58, P < .001) after adjusting for confounding variables. Conclusions: Our main findings showed that health literacy can facilitate improvements in physical activity and sedentary behaviors. Health literacy can facilitate health decisions for secondary prevention in older patients with CAD. Thus, healthcare professionals should assess patient health literacy and illness severity when designing secondary prevention programs.