To determine whether spa therapy has a beneficial effect on pain and disability in patients with chronic shoulder pain, this single-blind randomised controlled clinical trial included patients with chronic shoulder pain due to miscellaneous conditions attending one of four spa centers as outpatients. Patients were randomised into two groups: spa therapy (18 days of standardised treatment combining thermal therapy together with supervised mobilisation in a thermal pool) and controls (spa therapy delayed for 6 months: ‘immediate versus delayed treatment’ paradigm). All patients continued usual treatments during the 6-month follow-up period. The main endpoint was the mean change in the French-Quick DASH (F-QD) score at 6 months. The effect size of spa therapy was calculated, and the proportion of patients reaching minimal clinically important improvement (MCII) was compared. Secondary endpoints were the mean change in SF-36, treatment use and tolerance. One hundred eighty-six patients were included (94 patients as controls, 92 in the spa group) and analyzed by intention to treat. At 6 months, the mean change in the F-QD score was statistically significantly greater among spa therapy patients than controls (- 32.6 versus – 8.15%; p<0.001) with an effect size of 1.32 (95%CI: 0.97-1.68). A significantly greater proportion of spa therapy patients reached MCII (59.3 versus 17.9%). Spa therapy was well tolerated with a significant impact on SF-36 components but not on drug intake. Spa therapy provided a statistically significant benefit on pain, function and quality of life in patients with chronic shoulder pain after 6 months compared with usual care.