Context • Balneotherapy is one of the most commonly used nonpharmacological interventions for osteoarthritis (OA), but its mechanism of action in relieving pain and stiffness and in improving physical function is not well understood. Studies have found that therapy provokes a series of neuroendocrinal reactions with anti-inflammatory and analgesic effects. Sphingosine-1-phosphate (S1P), a bioactive lipid, has been implicated as an important mediator in the maintenance of physiological processes (eg, vascular barrier integrity) and in pathophysiologic processes such as inflammatory conditions. Accordingly, targeting S1P and S1P receptors may offer a potential therapy for arthritis. Objective • The aims of the present study were to determine whether (1) balneotherapy modified the circulating levels of S1P as well as some inflammatory parameters and stress markers, in patients with OA; and (2) to assess the relationship of those parameters to therapeutic efficacy. Design • This study was designed as an uncontrolled longitudinal study. Setting • The study took place at the Bolu Physical Therapy and Rehabilitation Hospital (Bolu, Turkey). Participants • Forty patients who suffered from general OA in at least 3 positions on the body, one of which could be the vertebral column, and who fulfilled the American College of Rheumatology Classification criteria and the Kellgren-Moore radiologic criteria, were enrolled in the intervention group in the study. Intervention • During balneotherapy, the participants were fully immersed in warm thermo-mineral water for 20 min at a temperature of 38°C to 40°C. A total of 15 immersions were performed in a period of 15 d. Outcome Measures • A baseline clinical evaluation of participants' pain, stiffness, and physical function was carried out using the Western Ontario and McMaster Universities questionnaire. Baseline serum levels of S1P, cyclooxygenase 2 (COX-2), matrix metalloproteinase 3 (MMP-3), and heat shock protein 70 (HSP-70) were measured using enzyme-linked immunosorbent assays and high-sensitivity C-reactive protein, with an immunoturbidimetric assay. The clinical evaluations and the biochemical measurements were repeated after completion of the balneotherapy period. Results • Balneotherapy caused a significant reduction in circulating levels of S1P and high-density lipoprotein and a limited increase in HSP-70 levels, in addition to a reduction in pain and stiffness and an improvement in physical function. In the Spearman's correlation analysis, S1P was found to be positively associated with serum levels of HSP-70, COX-2, and MMP-3. Conclusion • Balneotherapy modulated serum S1P levels in patients with OA. The effect of S1P modulation on the clinical outcome of patients with OA should be further investigated.