Rheumatoid arthritis (RA) is the most common autoimmune arthritis. The impact of chronic inflammation on atherosclerosis and insulin resistance has been observed in several autoimmune diseases. On the other hand, metabolic syndrome (MetS); a cluster of traditional risk factors for atherosclerosis and diabetes seems to be prevalent in RA patients. It is reasonable to think that protective factors against inflammation can protect patients against atherosclerosis and diabetes, too. Vitamin D (Vit D), a novel immunomodulator, is recently considered to play a protective role against cardiovascular diseases, insulin resistance, and obesity. This cross-sectional study was designed to evaluate the impact of serum Vit D on MetS and body mass index (BMI). One hundred twenty RA patients were enrolled. MetS was assessed according to Adult Treatment Panel III criteria. All patients with known confounders influencing Vit D serum levels were excluded. Serum value of 25-hydroxyvitamin D (25(OH)D) was measured using a commercial ELISA kit. Data were analyzed by SPSS software. A logistic regression analysis stated that prednisolone dosage [p = 0.028, β = 0.177, odds ratio (OR) = 1.194, confidence interval (CI, 1.09-1.32)], age [p = 0.002, β = 0.146, OR = 1.57, CI (1.05-1.27)] and Vit D serum levels [p = 0.049, β = -3.766, OR = 0.023, CI (0.001-0.978)] are all significant predictors of MetS occurrence in RA patients. It was shown that 25(OH)D is a protective factor against MetS. It was also shown that there is a negative correlation between BMI and 25(OH)D serum levels (P = 0.037, r (s) = -0.266). In summary, this study suggested that 25(OH)D plays a protective role against MetS in RA patients. However, this cross-sectional study did not permit a power calculation on the causal relationship between Vit D and metabolic syndrome. On the other hand, Vit D has a negative correlation with BMI in these patients.