Background: The Rotterdam Elderly Pain Observation Scale (REPOS) has not yet been validated for institutionalised cognitively impaired adults. To fill this gap of knowledge, we tested psychometric properties of the REPOS when used for pain assessment in this population. Methods: In this multicentre observational study, residents were filmed during a possibly painful moment and at rest. Healthcare professionals were asked to rate residents' pain by means of a Numeric Rating Scale (NRS)-proxy. Two researchers assessed pain with the REPOS and the Chronic Pain Scale for Non Verbal Adults with Intellectual Disabilities (CPS-NAID) from video-recordings. Results: In total, 168 observations from 84 residents were assessed. Inter-observer reliability between the two researchers was good, with Cohen's kappa 0.72 [95% confidence interval (CI) 0.64 to 0.79]. Correlation between the REPOS and CPS-NAID for a possibly painful moment was 0.73 (95% CI 0.65 to 0.79). Sensitivity (85%) and specificity (61%) for the detection of pain were calculated with REPOS ≥ 3 and NRS ≥ 4 as a reference value. Item response theory analysis shows that the item grimace displayed perfect discrimination between residents with and without pain. Conclusion: The REPOS is a reliable and valid instrument to assess pain in cognitively impaired individuals.