This study aims to develop an evidence-based, reliable, and valid health literacy index to evaluate health literacy levels among Korean adults for the Korea National Health and Nutrition Examination Survey collected by the Korea Disease Control and Prevention Agency. The index was developed over two phases and six, from January to August 2022. In the first phase, we defined the conceptual framework and generated the following items: 1) domains and item generation based on a conceptual framework, 2) content validity test with 18 experts in Delphi surveys, and 3) qualitative interviews with the general public (n=20) for content validity. In the second phase, we select and finalize items: 4) cognitive interviews (n=8) of the selected item for general understanding of the questionnaires; 5) a cross-sectional survey (n=1,041) for item reduction and validation. After 6) several rounds of reviews by and discussions with experts, ten items were selected for the final health literacy index: 3 items for disease prevention, 1 item for health promotion, 4 items for health care and 2 items for technology and resources. The index demonstrated good internal consistency (overall Cronbach's α=0.87; 0.72-0.78 for subtotals) and validity. This health literacy index can be used nationally to monitor the level of health literacy of the community and provide insight into health equity and national policies and strategies in the future.