Background: The aim of the present study was to assess the relation between metabolic syndrome (MS) and coronary artery disease (CAD) complexity, assessed by Syntax score (SS), and severity in non-diabetic patients with stable CAD who underwent coronary angiography, and to evaluate whether the MS defined by different definitions, including International Diabetes Federation (IDF) and American Heart Association/National Heart Lung Blood Institute (AHA/NHLBI) guidelines, similarly correlated with SS. Methods: The present study is cross sectional and observational with prospective inclusion of 248 consecutive patients (157 male) who underwent coronary angiography due to stable CAD. Results: The prevalence of MS was 54.4% according to IDF definition and 50.4% according to AHA/NHLBI definition. MS score according to IDF definitions (r = 0.446, p < 0.001), MS score according to AHA/NHLBI definitions (r = 0.341, p < 0.001) were moderately correlated with SS. In Fisher r to z transformation test the correlations of the presence of MS according to IDF and AHA/NHLBI definitions with SS were not statistically significant (p = 0.168, z = -1.38). The systolic blood pressure (p < 0.001, B = 0.354, 95% CI = -0.308 to 0.228), diastolic blood pressure (p = 0.006, B = -0.194, 95% CI = -0.333 to -0.056), age (p = 0.014, B = 0.147, 95% CI = 0.029 to 0.264), left ventricular ejection fraction (p = 0.031, B = -0.150, 95% CI = -0.286 to -0.014), waist/hip circumference (p < 0.001, B = 45.713, 95% CI = 23.235 to 68.1919) and log10 high density lipoprotein (p < 0.001, B = -22.209, 95% CI = -33.298 to -11.119) were the independent predictors of SS in linear regression analysis. Conclusions: MS is associated with the presence and complexity of CAD. Besides the presence of discrepancy in the limits of waist circumference, both IDF and AHA/NHLBI criteria were similarly correlated with CAD complexity.