Fibromyalgia is a chronic disorder of uncertain etiology, characterized by widespread pain, muscle tenderness, and decreased pain threshold to pressure and other stimuli. Obesity is a well-known aggravating factor for certain rheumatologic conditions, such as knee osteoarthritis. Emerging evidences are exploring the link between obesity and other rheumatic diseases, such as fibromyalgia. Epidemiological data show that fibromyalgia patients have higher prevalence of obesity (40%) and overweight (30%) in multiple studies compared with healthy patients. Several mechanisms have been proposed to explain "the hidden link", but at this time is not possible to ascertain whether obesity is cause or consequence of fibromyalgia. Among mechanisms proposed, there are the following: impaired physical activity, cognitive and sleep disturbances, psychiatric comorbidity and depression, dysfunction of thyroid gland, dysfunction of the GH/IGF-1 axis, impairment of the endogenous opioid system. In this article, we review the scientific evidence supporting a possible link between obesity and fibromyalgia, how obesity influences fibromyalgia symptoms and how fibromyalgia severity can be improved by weight loss. In addition, we analyze the possible mechanisms by which fibromyalgia and obesity interrelate.