Metabolic syndrome describes a complex metabolic risk factors including obesity, hypertension, dyslipidemia, and diabetes. This syndrome is diagnosed by medical conditions such as weight gain, high blood pressure, high blood glucose, and disturbance in lipid profile. Metabolic syndrome has become as an important and increasing global health problem, so finding potentially novel solutions with less adverse effects is favorable for health problems. Herbal therapy plays an important role for treatment of different diseases. Silybum marianum is a plant that is used for centuries as a herbal treatment in liver and biliary tract diseases. Silymarin is the main component of S. marianum and derived from fruits and seeds of S. marianum (milk thistle). S. marianum has been found to exhibit antioxidant, lipid-lowering, antihypertensive, antidiabetic, antiatherosclerotic, anti-obesity, and hepatoprotective effects. Therefore, the aim of this review is to summarize different animal and human studies regarding the effect of S. marianum in metabolic syndrome and to identify the underlying mechanisms of action.