Background <p>Phase angle (PhA) reflects fluid balance and cellular membrane integrity within the human body. Establishing normative reference values for PhA is crucial for research and clinical purposes.</p> Objectives <p>This study aimed to establish normative reference PhA values in a representative sample of the Korean population and determine the cutoff values for diagnosing sarcopenia in older adults.</p> Methods <p>This cross-sectional analysis included the data of 4,881 individuals aged 10–80 years. Multi-frequency bioelectrical impedance analysis was employed for PhA measurements. Sarcopenia was defined according to the Asian Working Group for Sarcopenia 2019 guidelines. The Lambda-Mu-Sigma method was applied to create centile curves and tables. Sensitivity, specificity, and area under the receiver operating characteristic curve were used to determine the sarcopenia cutoff points, and an association between PhA and sarcopenia was assessed using a weighted logistic regression.</p> Results <p>PhA values increased during teenage years and peaked in the 30s for both sexes, with a slower decline in women than in men from midlife. PhA was higher in men than in women across most body mass indices and age groups (<i>p</i> &lt; 0.05). The PhA cutoff points for diagnosing sarcopenia in older adults were 4.65^° and 4.25^° for men and women, respectively. A significant association between PhA and sarcopenia was found in both men (odds ratio [OR]: 6.63; 95% confidence interval [CI]: 2.97–14.79) and women (OR: 3.12; 95% CI: 1.50–6.48) after adjusting for confounders.</p> Conclusions <p>This study is the first to establish normative reference values for PhA across the lifespan of a Korean population aged 10–80 years and the cutoff points for diagnosing sarcopenia in older adults.</p>