Metabolic syndrome is a clustering of at least three of the following five medical conditions: abdominal obesity, high blood pressure, high blood sugar, high serum triglycerides, and low serum high-density lipoprotein (HDL).

Metabolic syndrome is associated with the risk of developing cardiovascular disease and type 2 diabetes. In the U.S., about 25% of the adult population has metabolic syndrome, a proportion increasing with age, particularly among racial and ethnic minorities.

Insulin resistance, metabolic syndrome, and prediabetes are closely related to one another and have overlapping aspects. The syndrome is thought to be caused by an underlying disorder of energy utilization and storage, but the cause of the syndrome is an area of ongoing medical research. Researchers debate whether a diagnosis of metabolic syndrome implies differential treatment or increases risk of cardiovascular disease beyond what is suggested by the sum of its individual components.

The key sign of metabolic syndrome is central obesity, also known as visceral, male-pattern or apple-shaped adiposity. It is characterized by adipose tissue accumulation predominantly around the waist and trunk. Other signs of metabolic syndrome include high blood pressure, decreased fasting serum HDL cholesterol, elevated fasting serum triglyceride level, impaired fasting glucose, insulin resistance, or prediabetes. Associated conditions include hyperuricemia; fatty liver (especially in concurrent obesity) progressing to nonalcoholic fatty liver disease; polycystic ovarian syndrome in women and erectile dysfunction in men; and acanthosis nigricans.

Neck circumference has been used as a surrogate simple and reliable index to indicate upper-body subcutaneous fat accumulation. Neck circumference of more than 40.25 cm (15.85 in) for men and more than 35.75 cm (14.07 in) for women are considered high-risk for metabolic syndrome. Persons with large neck circumferences have a more-than-double risk of metabolic syndrome. In adults with overweight/obesity, clinically significant weight loss may protect against COVID-19 and neck circumference has been associated with the risk of being mechanically ventilated in COVID-19 patients, with a 26% increased risk for each centimeter increase in neck circumference. Moreover, hospitalized COVID-19 patients with a "large neck phenotype" on admission had a more than double risk of death.

Metabolic syndrome can lead to several serious and chronic complications, including type-2 diabetes, cardiovascular diseases, stroke, kidney disease and nonalcoholic fatty liver disease.

Furthermore, metabolic syndrome is associated with a significantly increased risk of surgical complications across most types of surgery in a 2023 systematic review and meta-analysis of over 13 million individuals.

The mechanisms of the complex pathways of metabolic syndrome are under investigation. The pathophysiology is very complex and has been only partially elucidated. Most people affected by the condition are older, obese, sedentary, and have a degree of insulin resistance. Stress can also be a contributing factor. The most important risk factors are diet (particularly sugar-sweetened beverage consumption), genetics, aging, sedentary behavior or low physical activity, disrupted chronobiology/sleep, mood disorders and some medications, and excessive alcohol use. The pathogenic role played in the syndrome by the excessive expansion of adipose tissue occurring under sustained overeating, and its resulting lipotoxicity was reviewed by Vidal-Puig.