Background A proximal suprascapular nerve block has been suggested as an alternative to an interscalene brachial plexus block after arthroscopic shoulder surgery. The aim of this randomised controlled trial was to compare the analgesic and opioid-sparing effect of a low volume proximal suprascapular nerve block with placebo in patients with moderate-to-severe pain after arthroscopic shoulder surgery. Methods Patients with a VAS score equal to or above 50 during the first postoperative hour after planned arthroscopic shoulder surgery were included in the study. They were randomised to an ultrasound-guided proximal suprascapular nerve block with either 5 ml ropivacaine 7.5 mg/ml or 5 ml isotonic NaCl. Primary outcome was change in VAS score at rest from baseline to 30 min after the block procedure (T30). Secondary outcomes included total morphine consumption from 0-6 h after block procedure. Results There was a significant difference in mean VAS reductions at T30 between the two groups favouring the ropivacaine group (-50.2 vs -26.8, p < .001). Total intravenous morphine consumption from 0-6 h after block procedure was significantly lower in the ropivacaine group compared to the placebo group (8.5 mg vs 18.5 mg, p < .01). Conclusion In this study, a proximal suprascapular nerve block with only 5 ml ropivacaine resulted in a substantial pain reduction and opioid-sparing effect in patients with VAS of 50 or more after arthroscopic shoulder surgery.