Objective The ratio of apolipoprotein B (apoB) to apolipoprotein A1 (apoA1) has been reported to be associated with the metabolic syndrome (MetS) and to be a useful biomarker for future cardiovascular disease (CVD). However, the optimal apoB/A1 cut off for determining subjects with MetS has remained undetermined. We therefore assessed whether the apoB/A1 ratio can be used as an indicator of MetS and determined its optimal cut off for identifying subjects with MetS in a Korean population. Methods This study was a cross-sectional study. We collected the data regarding to conventional risk factors and serum levels of apoB and apoA1 in 10,940 subjects who participated in a routine health screening examination. Results After adjustment for confounding variables, the likelihood of MetS was significantly higher in the highest than in the lowest apoB/A1 quartile, both in men (odds ratio [OR]= 4.07, 95% CI=3.42-4.84) and women (OR=8.41, 95% CI=5.85-12.08). The optimal apoB/A1 ratio for detection of MetS in men was 0.65, with a sensitivity of 63.5% and a specificity of 61.3% (area under the curve=0.67, 95% CI=0.66-0.68, P＜0.001), whereas the optimal ratio in women was 0.62, with a sensitivity of 67.9% and a specificity of 61.9% (area under the curve=0.70, 95% CI=0.69-0.71, P ＜0.001). Conclusion The apoB/A1 ratio is independently associated with MetS in a Korean population. Ratios ＞0.65 in men and ＞0.62 in women are markers of MetS, independent of conventional risk factors. Measurements of apoB/A1 ratios may be particularly useful for determining MetS risk in clinical practice.