Background: There is no consensus on treatment for adhesive capsulitis of the shoulder. Physiotherapy is often used to increase range of motion but individuals may experience pain during mobilisation. Objectives: The objective of this study was to determine whether rehabilitation under N2O for shoulder AC improved pain and function more than rehabilitation alone (with placebo gas). Method: A randomised, 1:1 parallel arm, double-blind study in 4 university hospital outpatient centres. Adults with adhesive capsulitis for at least 3 months with intact cartilage were included. Participants were randomised in blocks of 4 and stratified by centre to receive 20 sessions of intensive physiotherapy over 10 days; the 10 morning sessions were performed with either nitrous oxide (intervention) or sham gas (placebo). Primary outcome: improvement in shoulder function at day 14 (D14) (Constant-Murley score). Results: Seventy-five participants were included, and data from 69 were analysed: 37 in the placebo group and 32 in the intervention group. Improvements occurred in both groups after the intervention. No significant difference in the Constant-Murley score was found between groups at D14 (median score increase of 12.0, IQR 5.6; 19.5 points in placebo group vs. 13.7, 5.2; 18.2 in the N20 group, p = 0.78). Pain score during sessions reduced in the intervention group from the first to final session (-11.6, p = 0.053) but not for the placebo group (-4.2, p = 0.414). Seven serious adverse events were recorded, 4 in the intervention group and 3 in the placebo group, with 11 minor adverse events only in the intervention group (mostly shortness of breath). Conclusion: Nitrous oxide gas associated with intensive physiotherapy for shoulder adhesive capsulitis did not improve function, pain or quality of life more than physiotherapy alone. zHowever, pain experienced during physiotherapy sessions appeared lower. Trial registration: Clinical Trial registration number NCT01087229.