Objective Metabolic syndrome (MetS) is associated with cardiovascular disease, which is the leading cause of mortality and morbidity. Hypernatremia and hypochloremia are also associated with an increased mortality. Thus, the aim of this study was to evaluate the association between the sodium-chloride difference (Na+-Cl-) and MetS. Methods In this cross-sectional and retrospective cohort study, we enrolled 3,875 subjects and evaluated the relationship between Na+-Cl- and MetS using logistic regression analyses. MetS was diagnosed according to the joint interim statement when a subject had three or more of the following criteria: hypertension; hyperglycemia; hypertriglyceridemia; low high-density lipoprotein (HDL) cholesterol; and abdominal obesity. Results There were 3,354 subjects without MetS and 521 subjects with MetS at baseline. The highest Na+-Cl- quartile (≥43 mmol/L) was associated with an increased risk of the presence of MetS compared to the lowest Na+-Cl- quartile (≤38 mmol/L) after adjusting for covariates, including age, sex, the body mass index, systolic blood pressure, fasting plasma glucose, triglycerides, HDL cholesterol, creatinine, uric acid and lifestyle factors [multivariate odds ratio (OR) 1.81, 95% confidence interval (CI) 1.17-2.84, p=0.0078]. After an 8-year follow-up, 658 out of 3,352 subjects were newly diagnosed with MetS. The highest Na+-Cl- quartile (≥43 mmol/L) was associated with an increased risk of the development of MetS compared to the lowest Na+-Cl- quartiles (≤38 mmol/L) after adjusting for covariates (multivariate OR 1.76, 95% CI 1.27-2.45, p=0.0007). Conclusion The sodium and chloride difference is associated with MetS.