Mud-bath therapy plays a primary role in the treatment and prevention of osteoarthritis that has been recognised since antiquity. Numerous studies have demonstrated its clinical benefits and its effects on inflammatory mediators (interleukins), the immune system, cenesthesic factors (endorphins), and the diencephalic–pituitary–adrenal axis. This study was conducted to assess the efficacy of mud-bath therapy with mineral water from the Sillene Spring at Italy’s Chianciano Spa in patients with osteoarthritis of the knee. Patients (n = 61) were divided into two groups. Group A underwent three cycles of mud-based spa therapy over a year’s time, whereas group B did not. Clinical conditions, visual analogue scale pain ratings, and Lequesne indexes of the two groups were compared. We also compared these same parameters in the patients of the two groups that were following the therapy with drugs and in the patients of the group A before and after spa treatment. The percentage of patients with no symptoms or mild symptoms was higher in group A than in group B. Within group A, this percentage was higher after treatment than before spa therapy. Even in the comparison between the patients of the two groups that were following the therapy with drug, the results was that in group A the percentage of patients with no symptoms or mild symptoms was higher than in group B. Statistical analyses based on various tests revealed that almost all these differences were highly significant. No adverse effects were observed in any of the patients in group A. In conclusion, the mud-bath therapy performed at Chianciano Spa with Sillene Spring water remarkably improved the clinical conditions of patients with knee arthritis and significantly reduces the frequency and severity of symptoms and the disability they cause.