Cutoff values for distinguishing smokers from nonsmokers have generally been derived from studies in White populations. Even though recent population-based studies have demonstrated that cutoff values can differ by race and ethnicity, few studies have explored cutoff values among Asian populations. We established the cutoff values for urinary cotinine and cotinine:creatinine ratio (CCR) using a nationally representative South Korean adult population sample. Data were obtained from the Korea National Health and Nutrition Examination Survey (KNHANES) 2008-2010 with pooled sampling weights. Adult participants (n = 11,629) aged ≥19 were included. Optimal cutoff values were determined by performing receiver operating characteristic curve analysis. The optimum cutoff values for urinary cotinine and CCR for Korean adults were found to be 164ng/mL and 1122ng/mg, respectively. The application of these urinary cotinine and CCR cutoff values for the female subpopulations aged 19-34, 35-54, and ≥55 years provided sensitivities ranging from 87.1% to 93.8% and from 82.9% to 94.9%, respectively, while maintaining specificity of ≥92%. For the 3 male age-based subpopulations, we obtained sensitivities and specificities for cotinine ranging from 93.1% to 94.5% and from 92.8% to 97.0%, respectively, and for CCR ranging from 90.0% to 96.9% and from 92.2% to 96.8%, respectively. Our cutoff values should allow researchers conducting environmental epidemiological or clinical studies in South Korea to distinguish adult smokers from nonsmokers effectively. However, different values may be applicable for subpopulations with different smoking prevalence rates or higher exposure levels to secondhand smoke.