Balneotherapy, either alone or in combination with mud (mud therapy or pelotherapy), is one of the most widely used non-pharmacological complementary treatments for various rheumatic diseases, including osteoarthritis (OA). Its beneficial effects likely stem from a combination of mechanical, thermal, and chemical actions. Balneotherapy has been shown to alleviate pain and stiffness, enhance joint mobility and improve quality of life. Most importantly, it may also play a role in preventing or delaying the progression of OA [13]. A growing body of evidence supports the therapeutic benefits of balneotherapy [14; 15; 16; 17], particularly mud therapy, on function, perceived pain, and quality of life in individuals with OA. These findings suggest that these therapies are effective and safe options for the clinical management of this condition [13; 18; 19]. This effect may be particularly due to its anti-inflammatory, antioxidant, and chondrocytes properties [20; 21]. Therefore, fortifying peloids used in balneotherapy with compounds that enhance their antioxidant and anti-inflammatory properties, such as polyphenic acids, could represent a promising strategy to augment the clinical effectiveness of mud therapy interventions. In general, polyphenic acids found in plants are being used as natural products with potential and proven health benefits. Among these, rosmarinic acid (RosA), although originally identified and isolated from _Rosmarinus officinalis_ L., is present in numerous species of the Lamiaceae family, where it has been reported to exhibit a wide range of biological activities, including anti-inflammatory, antioxidant, antidiabetic, antiviral, and antitumor properties, among others [22]. Due to its potential anti-inflammatory capacity, RosA has become a valuable natural resource for individuals suffering from chronic diseases such as arthritis or osteoarthritis, as inflammation and joint wear contribute to pain and reduced mobility. While some studies evaluating its clinical benefits have utilised RosA emulsions [23] or administered it orally [24], most of its applications have been assessed within the field of food sciences [22], particularly regarding the symptoms of osteoarthritis [25; 26]. However, there are no studies in the scientific literature regarding the use of RosA in the context of balneotherapy, either (pelotherapy or mud therapy) with or without peloids. Given the industrial interest in systematically maturing peloids to enhance the consistency of their production and composition throughout the year, while also improving their organoleptic properties, this study aimed to evaluate whether balneotherapy with a controlled-matured peloid, with or without fortification with RosA enhances clinical and functional outcomes. Specifically, clinical symptoms (pain measured using the Visual Analogue Scale, VAS), objective functional measures (assessed by knee flexion and extension), perceived functional status (evaluated using the Western Ontario and McMaster Universities Osteoarthritis Index, WOMAC), and quality of life (measured with the EUROQOL questionnaire) were examined in elderly OA patients after mud therapy interventions with or without RosA. Furthermore, to assess immunological safety and avoid an innate immunocompromised status due to the balneological interventions, the phagocytic and microbicidal capacities of neutrophils were evaluated, and the effects of the RosA-fortified peloid were compared with those of the non-fortified version. This pilot study, conducted within the IMSERSO thermal therapy programme in Spain for elderly individuals, represents the first investigation into the clinical effects (symptomatic and functional) of RosA in the context of balneotherapy. Understanding these effects, as well as ensuring the safety of its application--including the absence of immune compromise in participants--is crucial before undertaking larger studies that explore potential new mechanisms of action.