Background <p>Frailty in older adults is a multidimensional condition that extends beyond physical decline to include emotional and psychological vulnerability. However, evidence on how food security and overall diet quality vary according to frailty severity among older Korean adults is limited. This study aimed to systematically examine the associations between food security and diet quality, and frailty severity in this population.</p> Methods <p>A cross-sectional study was conducted using data from the Korean National Health and Nutrition Examination Survey from 2013 to 2020. Among the 62,686 participants, 9,263 adults aged 65 years and older were included after those with missing frailty index variables, no nutritional records, or extreme energy intake values were excluded. Frailty was assessed using a 41-item Frailty Index (FI), categorizing participants into nonfrail (FI ≤ 0.15), prefrail (0.15 &lt; FI ≤ 0.25), and frail (FI &gt; 0.25) groups. Diet quality was evaluated using the Korean Healthy Eating Index (KHEI), and food security status was assessed through a standardized questionnaire.</p> Results <p>The incidence of frailty was 14.1% in men and 23.8% in women, and it was associated with lower education, lower income, and living alone. KHEI scores decreased significantly with increasing frailty severity in both men and women (P for trend &lt; 0.0001). Severe food insecurity was significantly associated with frailty risk with an OR = 3.62 (95% CI: 1.23–10.66) for men and an OR = 9.03 (95% CI: 2.92–27.97) for women. In the case of KHEI scores, only frail women had a significant trend for the odds of frailty (P for trend = 0.0202), whereas no significant association was observed in men.</p> Conclusions <p>These findings emphasize the importance of sex-specific nutritional interventions and stable food supply systems for preventing frailty in aging populations.</p>