Various indicators have been suggested as replacements of body mass index (BMI) for estimating body fat percentage, including the recently introduced relative fat mass (RFM). However, RFM has not been assessed in different ethnicities; therefore, we evaluated whether RFM can be used to estimate body fat percentage in Korean adults and whether RFM is a useful indicator of obesity. Based on the Korea National Health and Nutrition Examination Survey (KNHANES) 2008-2011, we analyzed a total of 18,706 individuals (7,970 men, 10,643 women) aged ≥20 years who underwent dual-energy X-ray absorptiometry. We compared obesity (body fat ≥25% for men, 35% for women) misclassification rate of RFM (≥25 for men, 35 for women) and BMI (≥25 kg/m2). Diagnostic accuracy and optimal cut-offs of BMI and RFM were verified by comparing area under the receiver operating characteristic curve (AUC). RFM and BMI misclassification rates were similar obesity diagnosis based on body fat percentage (27.9% vs. 27.8%) among men. RFM misclassification rate was lower than that of BMI (22% vs. 45%) in women. AUC of RFM was higher in men (AUC: 0.79 vs. 0.78; p = 0.004) and lower in women (AUC: 0.80 vs. 0.83) than those of BMI (p < 0.001). In this study, RFM showed diagnostic accuracy for detecting excess body fat percentage, comparable to that of BMI. Using RFM with BMI could be beneficial in improving the diagnostic accuracy of obesity assessment in women.